+1 for allowing to add previously paid invoices to a project. It's crazy this seemingly straightforward and obvious concern for small business owners has not yet been resolved. As of July 2021, if I try to add a previously paid invoice (linked to an original estimate) to a project, I get the same dreaded error that all other users have mentioned. I tried to go about it another way and link the original estimate to the project first before linking the paid invoices and that was even worse. I can't risk all that information getting deleted and having to 1) spend a ridiculous amount of time recreating years of paid invoices or 2) messing up all of our books and reconciled transactions. Please Quickbooks create a fix for this!!!