I'm a parent volunteer and the treasurer of the school PTA. We are organising a large event to celebrate a significant school milestone and local businesses are donating and in return getting an ad in the event brochure. A few have asked for invoices and receipts which I have not previously been asked for and there is no recorded precedence for this in our files. I have not been able to find any clear guidance online and wondered if anyone knows whether its is OK to just create an invoice template and receipt? Or is there an acceptable alternative to these? Does doing this raise any tax / regulatory concerns? Advice would be gratefully received. 

Our first attempt to register as a charity failed as our constitution needed to be changed, so we are not yet registered (hope to try again in the next few months).

Thank you in advance.