。
How much will the postage come to?
Literal
Postal-fee [topic-は] how-much become-[polite-ますか]?
いくらになる ('come to how much, amount to how much') is the standard way to ask for a total — using なる ('become') rather than である ('be'), because the total is framed as a computed result. 郵便料金 ('postal fee, postage') is a formal compound: 郵便 (mail) + 料金 (fee, charge). Used at post office counters.