I'm presuming the OP is enquiring about sales using Simple Delivery.
If so, the answer is "yes and no". Buyers can (and should) use the basket, and group all of their purchases together in one transaction. (If they submit individual transactions, they will be charged full postage on each one by eBay)
If they use the basket properly, the postage will be combined, but by eBay, not by the seller. eBay will set the total postage price which should be lower than the total if the purchases were all made individually (but will typically be higher than the seller would have charged). So the seller no longer sets the combined postage amount.
Regarding invoices, I haven't had to issue one yet, but it appears that you will still be able to issue an invoice, but will not be able to amend eBay's postage methods or prices.