When shopping for fixed price items you need to use the Add to Cart button instead of BUY it Now.
Once you have added MORE than 1 item to your shopping cart, look at the top right corner for a Request Invoice link. In some circumstances the link is greyed out and unavailable, most of the time the link is active though.
If you're interested in just one item, use the contact seller link on the item's listing page, ask your questions, sellers often revise a listing to address buyer needs.
Auction purchases work differently. Most of the time you will need to contact the seller to ask for an invoice or explanation of charges. Some sellers have automated rules setup which will auto combine eligible items at checkout.
