In most cases, you can just do it. However, some sellers set their preferences to immediate payment required, so until that item is paid for, it will not be yours. That puts a crimp in paying for everything at once.
Do your research for this seller. Does he offer combined shipping? Personally, I would send seller a message, let him or her know you wish to buy several items, and want 1 invoice for the whole thing. Indicate the items you want. Even if he doesnt offer combined shipping, they may offer you a discount on multiple purchases for shipping. Can't hurt to ask.
