How to go about offering Priority Mail Express shipping

Description

Hello, internet!
I’m currently preparing a pretty significant listing and I am trying to be careful to get things set up correctly, particularly around shipping. I’ve picked up a Priority Mail Medium Flat Rate Box from the post office, and I want to use it to offer a 1-Day or 2-Day shipping option for free on my listing. I’ve found where I can pay the shipping costs, but I’m pretty confused about how everything works through PayPal. I can select Priority Mail Medium Flat Rate on the site, which appropriate charges me $15.05, but the delivery time is listed as 2-5 business days. There is no 2-day option that I can find. I see that there’s an option to select Priority Mail Express, which is listed as 1 business day, but I don’t have the right box for that, right? On the USPS website, I only see envelopes for Priority Mail Express, no boxes. Can I pay for Express shipping and use a standard Medium Flat Rate Box? Do I have to get a different box? How does the transaction work on PayPal?
If someone could help me sort this out, I would hugely appreciate it. Thanks!