The problem we had selling on Amazon Marketplace was when someone bought multiple items. There is no facility to discount postage and Amazon were charging some ridiculous amount per item - actually I reckon it was £3.50 - that we could not alter. Your best bet is often to email the seller and ask them if they have a discount available. Most do.
I'll be honest, unless it is actually sold by Amazon, though, I rarely buy from there.
-------------------- I loved my grandmother very much.......and she fetched a good price on eBay.