Trade Counter sales to Credit Account Customers

The 'Cash Sale' process seems to offer a reasonably streamlined process to order goods, accept payment and fulfil them (i.e. the customer takes the goods away).
However, we are being told the best process for credit account customers to take goods (but pay on their credit account) is to issue an Invoice.
This is a single process, but does not have any provision for printing any kind of collection note or taking a signature (to prove the customer has taken the goods).
We are looking at a 3rd party plugin to capture signatures electronically, but we are being told that customising the Invoice template to include the collection signature would then customise all invoices sent (as part of the weekly / monthly invoice run) - which is not practical.