Re: Adding invoices as income on Quickbooks self employed.
I have let @EmmaM know you've replied.
I had a thought of a different route you could take it, one which removed the need to snap anything to the transactions. As Emma mentioned, the facility to have the paid invoices affect the business income, have you tried to create a manual csv of all your cash income? For example, when your students pay cash; mark it down on an excel document on your mobile device or note the amount and date somewhere to later be added onto an excel document.
You could then take the excel document, put the Date they paid you in a date column, put Cash Payment from #name (as an example) in the description column, and put the amount in the amount column as a positive number. When you save it as a .csv file; you could then import this supplementary to your banking connections feed of transactions. If you'd like to know more about this option, Let me know and I'll elaborate on any questions you have.
I'll check in with Emma and she may post herself, if not I'll send another message shortly.