If it was me and someone told me the 4 items he wanted to purchase, I would combine everything he wanted to purchase into the one listing he messaged me on, (making sure to cancel and delete the other 3 listings). That way the buyer can be sure that he will get the pricing and mailing cost you both agreed to.
Alternately, the buyer can purchase the 4 items, but before he pays, he can message you to modify the costs on the invoice before he pays. This doesn't work if you have immediately payment required.
Good luck.