It depends upon the seller, so it would be best to ask before purchasing any items. Many sellers combine items with a lower shipping cost, some may combine with the same shipping cost as for one item. If they agree to a combined/lower shipping cost, you should wait for their
ebay invoice to pay. Some may not agree to the combined lower shipping request.
As for requesting to start a new listing that combines the lots of interest, you may request it, but I think most would not do it. I takes more time for them to change/combine the listings, and since they may be paying for the listings, it can cost them more money to do it.
Once you get a response (email) from the seller, if they choose to not follow what they agreed to do, you can first "remind" them to see if they will change to follow the agreement, and then escalate to
ebay as a problem with the listing(s) to resolve the problem (and include the email evidence).