Stop Sending Unprofessional Invoices
A professional invoice is always accurate and arrives on time. It contains clear line items, correct tax calculations, and a unique invoice number.
The challenge is achieving this consistency when billing rules are complex. Generating an invoice often requires pulling data from multiple systems like a time tracker, an expense log, and a project management tool, then manually applying the correct rates and taxes.
We built an automated invoicing system for a 7-person creative agency. They were spending 10 hours a month creating 80 invoices from Harvest and Asana data. The system we deployed generates and sends each invoice in under 10 seconds, and they have not had a single billing error in 6 months.
What Problem Does This Solve?
Many businesses use QuickBooks Online for invoicing, but it cannot create invoices from external data automatically. You have to manually key in line items from a project management tool or a timesheet. This copy-paste process is where tax rates get forgotten, hours are mistyped, and discounts are misapplied.
Consider a 15-person marketing agency billing clients based on billable hours from Harvest, ad spend from Google Ads, and a fixed monthly retainer from their HubSpot deal record. The account manager exports three reports, opens an Excel template, and manually combines the data. This takes 20 minutes per invoice and is so error-prone that a second person has to double-check every single one before it goes out.
These tools are designed as systems of record, not systems of operation. They can store an invoice, but they cannot enforce the business logic required to create it. Zapier can move a number from one box to another, but it cannot handle tiered pricing or conditionally add line items based on project tags, forcing you back to a manual process.
How Does It Work?
We start by connecting directly to the source APIs of your business tools, for example, Harvest for time tracking and HubSpot for client contracts. We use Python with httpx to make these API calls asynchronously, pulling all necessary data in under two seconds. This raw data is temporarily stored in a Supabase database table for processing.
The core logic lives in a Python function deployed on AWS Lambda. It reads the raw data, applies your specific billing rules, and constructs a structured invoice object. For a client who needed to parse project notes to create line items, we used the Claude API to extract billable tasks and quantities from unstructured text, achieving 99% accuracy after training on 50 examples. The entire transformation process for one invoice completes in about 8 seconds.
The AWS Lambda function then calls the QuickBooks Online or Xero API to create the invoice draft directly in your accounting system. The final invoice includes formatted line items, sub-totals, and the correct tax codes pulled from the client record. This eliminates all manual data entry into your accounting software. The total cost for this AWS infrastructure is often under $20 per month.
We use structlog for structured JSON logs, which are sent to AWS CloudWatch. We set up alerts that trigger a Slack notification if an invoice fails to generate for any reason, like an expired API key or an unrecognized client ID. This ensures we know about a problem within 5 minutes, long before your client or accounting team does.
What Are the Key Benefits?
From 20 Minutes to 8 Seconds
Generate and send a complex, multi-source invoice in under 10 seconds. Free up hours of administrative time each month.
Zero Manual Data Entry Errors
By pulling directly from source APIs, we eliminate copy-paste mistakes. No more corrected invoices or awkward client conversations.
You Own the Billing Logic
We deliver the full Python source code to your GitHub. Your billing system is a company asset, not a dependency on a SaaS vendor.
Alerts for Failures, Not Silence
The system monitors itself. If an API connection breaks, you get a Slack alert with the error details instantly.
Connects Directly to Your Tools
We build integrations for your specific stack, whether it is Asana, Trello, Harvest, or custom internal software.
What Does the Process Look Like?
Week 1: Scoping and Access
You provide read-only access to your source systems and walk us through your billing rules. We deliver a detailed data flow diagram.
Weeks 2-3: Core System Build
We write the Python code for data extraction, business logic, and API integration. You receive progress updates and access to the code repository.
Week 4: Deployment and Testing
We deploy the system and connect it to your accounting software in sandbox mode. You review and approve the first batch of 25 generated test invoices.
Post-Launch: Monitoring and Handoff
After you approve testing, we go live. We monitor the system for 30 days to ensure stability and then hand over full documentation and a runbook.
Frequently Asked Questions
- How much does a custom invoicing system cost?
- Pricing depends on the number of data sources and the complexity of your billing logic. A system pulling from two standard APIs with straightforward rules is a faster build than one parsing unstructured data from five sources. After a 30-minute discovery call, we can provide a fixed-price proposal.
- What happens if a source API like Harvest is down?
- The system is designed to be resilient. It will attempt to connect to an API three times over a 15-minute period. If it still fails, it will halt processing for that specific invoice and send a Slack alert with the details. This prevents bad data from entering your accounting system.
- Why not just use a tool like Chargebee or Stripe Billing?
- Those are excellent for recurring SaaS subscriptions. They are less effective for service businesses with complex, non-recurring billing based on hours, expenses, and project work. Our system is built for that specific use case, integrating with your project management and time tracking tools to automate bespoke invoicing.
- What if our billing rates or tax rules change?
- The business logic is contained in a well-documented Python configuration file. Simple changes like updating a rate can often be done in minutes by any developer. For more complex changes, we offer flat monthly maintenance plans or can scope a small follow-on project.
- Can it handle invoices that need manual review before sending?
- Yes. The standard workflow creates a 'Draft' invoice in QuickBooks or Xero, not a 'Sent' one. This gives your team a final checkpoint to review the automatically generated invoice, make any final adjustments, and then click send. We can also configure it to send automatically if you prefer.
- What kind of access do you need to our systems?
- We only require read-only access to source systems like your CRM or time tracker. For your accounting software, we need permissions to create draft invoices. We use dedicated API keys for all integrations, which you can revoke at any time. We never need user logins or passwords for your staff.
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