How do I charge customers the exact amount shipping costs?

[PureSoaps]

For orders of $47 or more I want to offer free shipping. 

But for every order below $47 I want to charge customers the exact same amount USPS is charging me for the shipping.

When I go to shipping settings, there's already a bunch of rates filled out there, but I have the option to delete them. If I delete them, will it revert to just charging users whatever it costs with USPS?

If not, how do I set it up so that users pay exactly the correct amount each time?

[MOISCHKOWITZ]

I think you should set the conditions based on weight and make it exactly like they charge at UPS. 

[PureSoaps]

The issue is that I don't know exactly what USPS is going to charge me. So I'm afraid if I overestimate, then I'll be charging users too much money. If I underestimate I'll not collect enough to cover shipping.

Is there any way to just have the user pay exactly whatever USPS is charging on an order by order basis?