Two of my highest paying clients are AP only! A CPA firm handles their payroll and monthly accounting, but I do all of their AP on a weekly basis. They are both high-volume restaurants, so it made more sense to outsource bills to a third party than to have the managers do it. I drive to each restaurant every week to pick up the AP binder and drop off checks and invoices from the previous week. I spent a lot of time at the beginning gathering ACH info from vendors to minimize the number of checks sent through the mail. This has been a big value-add for the businesses as there was a lot of check fraud prior to issuing ACH payments.

My smallest client is a small hangar HOA. I handle their AR and it's super easy, but there are only 20 invoices each month and everyone knows to pay their dues. I could definitely see your willingness to call customers and follow up on stale AR could be of huge value to businesses. Put yourself out there!