Do you plan on shipping additional items for FREE then when you combine shipping?
If not, it is basically saying...ok, you're going to charge $5.00 (for example) for the first item, if someone purchases 2 of these items, what "additional" charge are you going to charge them? 50%? ($2.50)
more?
less?
a percentage?
When you list in quantities, you need to set your parameters as to how you want ebay to configure the "bill" for your customer when they make multiple purchases. It is typically best to specify as much information in the listing and have all your details configured so your bidders know ahead of time, this way you are more likely to sell your items, and for possible more if people know "what they're getting into" as far as fees and shipping go. Most folks don't like bidding in the "blind"...meaning that "fees and charges" are configured once the auction has ended and the seller now packs the items and determines the price. Most people are leary of these auctions as that is how "some" sellers stick it to the buyer by hitting them with outrageous fees and the buyer can either suck-it-up and pay them, or get a NPB filed on them.
Either way, it is best to determine your fee structure for your auctions to eliviate the possibility of "issues" arising at the end of the auction.
If you leave the additional item amount blank, it will most likely show up as FREE when ebay goes to configure the bill...so fill in as much as possible!
I hope that answered your question
Good luck,
Kathleen