Why would you use post instead of get? There is no need to change
anything really (if your code is designed right at least) because all
you would need to do is "/uri/:category_id/:product_id" =>
"controller#action" and it will route it and setup the params the same
way it would with query strings. The problem comes in with the fact
that you can kill your SEO by doing that because you provide no useful
information for the search engine, you actually strip some context but
that's not my concern it's yours.