Triton Testing Surprise

[Merv49]

On 31 March I mailed in my fifth Triton test as I always have. I put three stamps on the box and dropped it at the post office. Today I got the box back with a sticker on the front saying "RETURN TO SENDER, REFUSED, UNABLE TO FORWARD. Needless to say I was surprised and had to go to the post office to get some answers.
Turns out the my test did go to California where additional postage was requested from Triton who refused to pay so my test was sent back to me. Turns out that the little box is considered a parcel and costs $3.50 to mail and not just a few stamps on the box. At least that is what the post office told me. I looked online at one web site and the instructions say to put a stamp on the box just like a letter but now that may be bad information. I paid the $3.50 and re-mailed my test.
Anyone else have this problem or have I missed something and just got lucky on my first four tests that wen through with no problem?

 

[jsker]

Thank you for the information

 

[Norcalreef]

I’ve only sent one, but it costs $3.50 to ship. Thought never crossed my mind to use stamps, I associate stamps with mailing letters etc. I just had my wife mail it however the post office said to.

 

[DHill6]

Paid $3.50 to ship the latest box. Back when they started up through Unique, they were pre-paid envelopes to ship back. As long as it was placed in the mail, made it fine. If you happen to have a certain postal employee that forced it through the envelope size ruler and touched the side, it would cost extra postage. Lesson there, put it in a drop box.