Nobody made you pay the full delivery cost for one parcel instead of four. You made a personal choice to do so by paying on the combined invoice. While you have no right to demand any discount if one is not offered you are at liberty to uncombine the items, preferably one ot two being sent to friends addresses, and making 4 different payments. The seller would then have to pay in full for each item too be sent and would also pay an extra 90c in Paypal fees.