Can I drop off a prepaid package (priority mail: large envelope) in a blue USPS collection box no matter how much it weighs?

Yes, you can. When it comes to online labels and the blue drop boxes, "If it fits, it ships."

The online label makes you a "known shipper." The 13-ounce weight limit applies only to pieces with postage paid entirely with stamps.

@center: I meant if it fits in the mailbox, you can mail it there.

From the IMM:

235 Mail Entry and Deposit

235.1 Place of Mailing

235.11 Items Eligible for Deposit or Pickup

Priority Mail International items bearing a computer-generated customs form with customs data that has been electronically transmitted (e.g., using Click-N-Ship service, an authorized PC Postage vendor, or the USPS Web Tools system) may be deposited through any of the following methods, provided postage is paid by a means other than the use of postage stamps:

In a private mailbox bearing a return address that matches the address at the point of pick up, when the customer or business is known to reside or do business at that location.

Through Pickup on Demand service.

Through Package Pickup service.

At a Postal Service retail counter.

Into a Postal Service lobby drop.

In a collection box.

At a Contract Postal Unit (CPU).

At a USPS Approved Shipper location.

http://pe.usps.com/text/imm/immc2_015.htm