Use a calculator. The discounts usually only apply to the item price, not postage, so if an item is $120 with $20 postage, the discount only applies to the $120. It's easy enough to check on a calculator. Using the $120 amount for a 20% discount, enter 120 x 20 %. That will show a total of $24. Subtract the 24 from 120 to get your final amount after the discount. 

Just remember that with eBay generated discounts, like the 10% one on Boxing Day, you often have to spend a minimum amount. If you don't have enough from one purchase, you can add items from other sellers too. Put them all in the cart and then pay for them in one transaction. Usually they say maximum of 10 items per transaction, but I've had 30+ in one transaction before and the discount applied to all of them.