FEE TEMPLATE: buyers have problems with the postage

I have several buyers who ask me to send an invoice, even they have got an invoice direct from Delcampe.
I also have buyers buying 1 item, and then pay the invoice.
After some minutes they buy another item and pay at once with one more postage.
For 2 days ago a buyer bought 2 items and pay the invoice, after about ½ an hour he bought 1 more item and paid the invoice with postage.
When I later look at my auctions again, the buyer had bought and paid 6 items on 5 different invoices all with postage.
I have contacted him and refund postage for US$ 18,-.
At the moment I have a lot of buyers with an unpaid invoice, and I simply don't know, if they are awaiting an invoice from me, even they should have got an invoice direct from Delcampe.
Other of you sellers with this problem? and have you contacted Delcampe about this problem?
Another problem on this Template is the number of items for postage.
I have a number of 12 items for the max weight of 100 gramm, but 12 small air mail covers only have a weight of perhaps 55 gramm.
What to do if a buyer buy perhaps 20 items and the weight still is lower that 100 gramm.
Of course I can change the postage if the invoice isn't paid, but often a buyer pay at once, and then I can't do anything.
Any proposals?
Benny

De informatie is niet meer beschikbaar omdat de rekening van deze gebruiker is verwijderd (AVG)

It's quite simple, ignore the templates and as Baltija says, adjust your settings so that customers cannot pay without you sending an invoice.
It might seem a bit more work at the time ( a few seconds in reality) to send an invoice 'manually' when you've established that your buyer has stopped buying, but that is a lot less than trying to sort out a mess with overpaid postage and arranging refunds. (Doesn't always work, though! - some buyers will always find something else they want after having paid )

The templates are produced by computer programmers, not people who understand stamps and covers and the infinite variety of possibilities. Yes, they are great if everyone just sold exactly the same thing, but that doesn't happen.

De informatie is niet meer beschikbaar omdat de rekening van deze gebruiker is verwijderd (AVG)