Sometime customer does not check the packaging properly and jump to the conclusion that invoice was not sent away. mention that u send invoice in every package and then apologies if u missed enclosing one and they did not receive . Attach a copy of invoice to email anyway. Point to that.
Then offer if they want a hard copy of it as well, u will post it.
Add to that you are not vat register so u can't provide vat receipt. Hope it helps.