no, you are wrong. no such 規定.

never hear that 郵件厚度不夠，升級成priority mail是郵局的規定,

this is not right. no such 規定.

www.usps.com

First-Class Mail :

First-Class Mail gets your mail there fast.

Postcards, letters, large envelopes (flats) and small packages can be sent using First-Class Mail®. This service, along with Priority Mail and Express Mail, is ideal for sending personal correspondence, handwritten or typewritten letters, and bills or statements of account. It may also be used for advertisements and lightweight merchandise. First-Class Mail prices are based on both the shape and weight of the item being mailed. For items weighing more than 13 ounces, use Priority Mail.