You don't need $30/month accounting software to send a professional invoice. You need a template, your client's data, and a PDF. That's it.
If you used AgentForms for your initial client intake, all of this information is already collected. Client name? Already in the system. Address? Already there. Scope of work? Already documented. The invoice generates from the same data you used for your quote.
QuickBooks, Xero, FreshBooks — they charge $20-$50/month for features you don't need. AgentForms generates the invoice documents. You handle bookkeeping in a spreadsheet or your accountant's system. Separate the document generation from the accounting — it's cheaper and simpler.