In those types of situations what i try to do is simplify the process by simply revising the item price or shipping directly in the listing before the purchase is made. This works well in cases where good 2 way communicatiion has been established.
For example in the situation where a customer hss purchased one item, afterwards they decide there is a second item they like but they are concerned about getting a price break on combined shipping so they message me asking what to do. Often I will revise the item in question to free shipping (or whatever amount was agreed on) then message that the shipping charge has been adjusted for their purchase. In the message I inform the buyer the item will remain free shipping for only a few hours, after that it will be revert back to normal pricing. Works well, cannot recall anyone not purchasinng pretty quickly afterwards, no chasing invoices required.
I will only do this with customers who communicate well.
