Are you sure they don't email the shipping stickers?
Given current costs for long distance telephone service, I wouldn't stress too much about a non-toll free call.
Does your invoice not include instructions about returns?
“Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”
Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2