I'm just speculating on how it could have been done. Even without a mobile phone, Swampy could have used his camera (that we saw on April Fool's Day) to upload pictures to Kinko's. Assuming the shop is close, Swampy could have run over while they were carrying things out to the truck. With everybody going in and out, Doc and Sandy might not have noticed Swampy's absence, or they might have assumed he was trying to avoid work.
Of course, now the question is why didn't Sandy tear it up and feed it to Swampy, and maybe Jake?