[riezebos]

As a freelancer, I use Moneybird[0] for this purpose. There is a free tier up to 4 invoices, 10 incoming invoices and 10 bank transactions per month. Most months I fit in the free tier, some months I pay for 1 month of subscription.

Maybe a model like this would work for you as well? The users you are replying to might switch and sometimes pay if they have a busy month.

[0]: https://www.moneybird.com/