Why when I combine postage dose it turn around I put express post as the top option at the standard postage price?

[dvd.dj]

When buyers purchase multiply items and have requested a combined postage invoice, ebay seems to some times put express post as the top option. Even if I change it, the buyer in this case has ended up with an incoice for express post at $7.50 although you can not buy an 500 gram express post bag at this  price anywhere. Now this buyer is threating to leave negetive feedback and keeps emailing me, like she wants a fight. I have refunded her the entire purchase back. How do I stop this from happening again? My listings first come from Auctiva