"Shall I pay now" is technically correct, but has slightly different connotations than "Do I have to pay now?" . "Do I have to pay now?" is a completely open-ended inquiry; it does not expect any specific answer. Whereas "Shall I pay now?" slightly expects the answer to be "Yes", because "Shall" implies definitiveness. When you say you "shall" do something, it is similar to saying you "will" do something. "Must I have to pay?" is not correct, because "must" and "have" are redundant. "May I pay now?" is asking permission to, not whether you are going to or not, or whether you can or not.