I find this odd.  I have just looked at some of your listings, and none of the ones I saw said "immediate payment required"  I then checked out a few other sellers, looking for any that required immediate payment, and next to the Paypal logo, under "payment information" it is there, but definitely not on yours.  I don't have immediate payment selected either, yet some buyers had claimed they had to pay straight away.  I thought maybe eBay was targetting this to specific buyers, maybe because they were slow payers in the past. I have successfully been able to send a combined invoice to other sellers without any problem, except for a couple of times when they went ahead and paid, and I just refunded them part of the postage and combined the items, none of these had mentioned or complained that they were forced into immediate payment, so I just presumed they didn't know otherwise. Luckily, I haven't had a problem with eBay adding "best offer", although I have noticed, when listings come to an end and need to be relisted, that "best offer" is ticked by default, and I just go ahead and change it before I relist.