Stamped envelope rates?

I recently added a product that can be shipped in an envelope for 55 cent stamp, or 1.55 international. I’m wondering if theres an option to factor this into the api rates or if not, what would be a reccomended approach? So that if only this one item is ordered shipping rate would be accurate, or if they ordered along with normal items that use usps package service it would charge the regular rates.

I am a small crafting bussuiness so I am using mostly free plug ins. USPS is the only carrier I am using. TIA