Oh please no. I have to drive 10 miles one way to pick up a priority mail package the day after it is delivered. I live out in the sticks, and they won't deliver directly to houses, and our mailbox center won't accept them! Just put a note on the standard shipping that once it leaves your site, you can't guarantee that it will arrive on time. Also, you could add a shipping
insurance option and state that if you don't chose that option with standard shipping, if the package is lost, you can't replace it or something else to that effect so you're not constantly losing revenue having to replace lost packages. Just ideas.