CASTLE ROCK, Colo. -- A Castle Rock man has been warned that reusing a United States Postal Service "Priority Mail" cardboard box is against federal law.

Gary Adler said he was just recycling a box that was going to be thrown in the trash, but the Postal Service said that kind of repurposing is illegal.

Adler uses the boxes to mail sports memorabilia for his nonprofit organization Pro-Players Association.

"We recycle old boxes that we get at the grocery store or from other merchants, and Dumpster dive sometimes," said Adler.

Sometimes, Adler used the "Priority Mail" boxes that were left in the trash near the P.O. boxes.

"We took off the tape and we took off the old label that was on there originally," said Adler, describing a box he recently sent that was returned by the Postal Service.

"And we re-taped this box that was originally this way and we made it this way," showing how he turned the box inside-out so it's brown on the outside and not white.

But the Postal Service said what Adler did is against postal regulations. He is being warned not to do it again, but if he continues to reuse "Priority Mail" boxes, he could be charged with misuse of postal property.

"Our Priority Mail and Express Mail boxes are, bottom line, supposed to be used for that service," said Nicole Reiter with the Postal Service. "That is what they are intended to be used for."

The Postal Service said it promotes recycling, as long as customers pay accordingly, even if the box is turned inside out.

"It is important that the customer uses it for the proper service," said Reiter. Reiter said customers sometimes order their priority and express mail boxes online and then try to use them standard mail. She said that makes it difficult for the USPS to determine which boxes are new or reused.

"I think it is stupid. Tentatively, this box was on its way to the Dumpster at the post office," said Adler. "Here, the post office is saying you can't use our boxes for recycling, go find something else."

The U.S. Post Office refused to accept Adler's recycled "Priority Mail" boxes. Adler said because of this incident he will no longer use the U.S. Postal Service to mail his packages.

Reiter said the boxes are a cost to the Postal Service and it supplies them for free to customers who pay for priority or express mail. She said enforcing the no-recycling rule would keeps postal rates low for everyone.