To clarify my thoughts...
Yes, Donna, you got it. As a buyer, if there are 2 items a seller has & they require payment immediately, but offer combined shipping only if paid in one invoice, I don't think the seller actually thought that out. A buyer would have to pay for each item, thru eBay (I don't use this checkout), individually. This means no combined shipping for the buyer & the seller is hit with 2 paypal fees & a ticked off buyer that thought you offered combined shipping, but because you require immediate payment, you can't get the combined shipping. It just doesn't make sense.
Unless I only want one item from a seller, I move on. Think about this... maybe I see a $200 comforter that I want to snatch up today, but I won't have enough in paypal until 5 days from now, you loose my business. If I decide to take the item & paypal debits my credit card for what I don't have in paypal, you are charged more for the credit card charge.
I have sold a long time on eBay & can offer a few wise words of advice... be as flexible as you can be, within reason. Doesn't mean bend over & take it from unresponsible buyers, but be flexible. Too many limitations upfront scare off buyers & this includes the good ones, like me. If I see too much negative non-paying language or threats in an ad, I pass you by. I personally use paypal, but there are many buyers who don't trust the company or the internet. A seller cuts there own throat, lowers bids & sell-thru ratios the more limitations they impose on buyers. Same with International shipping. I love International bidders. I love unconfirmed buyers that ship to a gift location as well. I can count on one hand out of over 10,000 (much higher actually) paypal transactions, the number of chargebacks I have had. The benefits of being flexible far outweight the negatives.
It doesn't matter what format you sell on eBay, one rule applies, K.I.S.S. Simply...More equals more. Offer MORE flexibility & your revenue will increase, inevitably!