The address_line 1 is not necessary to get an accurate quote. However, it is necessary to generate a label that you'll need for sending the package. You'll have to be careful to enter the correct address. FedEx charges extra afterwards for address corrections. Our goal was to build an API that allows you to get accurate quotes without having to enter a lot of data. However for the final label purchase the exact address will be necessary.
UPS/FedEx etc. don't allow you to pass on your account to people that are not directly connected with your business. I assume that is what you meant by "messy." If you still want to do that stripe connect is certainly a good choice. This is exactly what we're working on - to get contracts with courier services to eliminate this problem.
About the flow that we had envisioned for our service: customers choose the product they want to buy -> tell us in which country they live -> we show them all available courier services -> they chose the servicelevel that suits their needs best (fastest, cheapest, whatever) -> check out -> the label is directly sent to the vendor -> vendor sticks it on the package -> brings it to the post office/gets pickup (vendor can specify if only pickups are allowed & which day is best for pick up) -> done (all through API)
Just email me at laura(at)goshippo.com if you have any more questions. Happy to help out.