How to say “I'll check the postage at the counter before you pay; this envelope is heavier than it looks.” in Japanese

In Japanese

この封筒(ふうとう)、見(み)た目(め)より重(おも)いから、払(はら)う前(まえ)に窓口(まどぐち)で郵便料金(ゆうびんりょうきん)を確認(かくにん)してくるね。

Kono fūtō, mitame yori omoi kara, harau mae ni madoguchi de yūbin ryōkin o kakunin shite kuru ne.