Invoicing. I sell a product that customers use on many of their own projects, and each project of theirs means one invoice for me. I can solve some of this with automation, and I will get around to doing this, but it will remain hard to keep on top of even then because the nature of the thing I'm selling relies on the customer to validate that the tally is correct before I send the invoice.

(I also have the opposite problem when the customers actually pay, because they often batch many invoices together into one payment, which makes reconciliation surprisingly difficult. I wrote a little script in Python to make that easier - sort of along the lines of "Given this total, which of these invoices could potentially satisfy it?" - but it's still frustrating.)

If the business continues to grow, I anticipate hiring someone to take care of this aspect for me in its entirety, because it's demonstrably a poor usage of my time, and I derive zero pleasure from it.