eBay

A buyer has brought 7 items so will send in 1 box but cannot apply a discount for reduced postage as I had items with free postage, how can I sort this out

I have had a credit of $8.05 for a few years from a cancelled postage, I thought the only way to gain it back was to sell again on Ebay but no luck, sold an item but was charged the full amount of fees and still have a credit of $8.05 how can I ever ...