Add USPS Shipping Large Envelope

Exactly which USPS service are you talking about (give the *full* name!), and where do you see that Ubercart doesn't include this. It can perhaps be added, but we need more details to go on.

I guess this was either not an available service, or was overlooked, when the USPS module was first developed several years ago.

If you have a patch to offer then we can consider it. However I doubt that's really what you need. Ubercart has no way of telling whether your envelopes will be large or small, or if large whether they will be machinable (e.g. rigid or odd shaped) and charged as packages, not letters. But that's not a problem, as the whole point of shipping calculations is ONLY to figure out what to charge your customer. You are always free to choose your package and shipping method at shipping time - this is not something that needs to be known at purchase time. If you simply charge your customer for a First-Class Parcel, you should be fine. If that's charging too much consistently, you can always apply a percentage rate markup to reduce the amount you charge your customers. Or you could just configure a flat rate, as most of your packages probably cost the same amount. Or a flat rate proportional to the number of items purchased.

What happens if they order more than one say - they order six?

won't you be over charging for postage?

Shipping is being refactored for 8.x-4.x. It's unlikely that this will be backported to 7.x-3.x.