My take on WOPA

The niche is the whole win here. Not "invoicing software." Invoicing for UK tradespeople, inside WhatsApp, where they already live all day. A sparky is not going to open a spreadsheet on a ladder, but he will type "invoice Chris £850 rewire" into WhatsApp in five seconds. That sentence is your best ad. Use the literal message as the hook in half your posts, because it shows the product working before anyone has to think.

The payment chasing is the feature that actually gets people to pay you, so it should get its own posts. Every tradesperson on earth hates ringing a customer about an unpaid invoice. WOPA does the 7, 14, and 28 day reminders automatically, and "chase both" in one message is the kind of thing that makes someone screenshot it and send it to their mate. The fact that the customer does not even need WhatsApp, they just get the email PDF, removes the only objection a skeptic would have.

The voice of the product is a gift, do not lose it. "Done." "Sent." That clipped, no-nonsense tone is exactly how this audience talks and it builds trust faster than any feature page. Carry it into the tweets. Short lines, no fluff. And the founder pricing, £9.99/month locked for the first 200, is real urgency that is actually true, so say it plainly and let the count do the work.