Can you ship a 2.25" by 3.5" envelope? Will it cost any extra?

Question

It will weigh hardly anything (It will contain 1 1.5" pin-back button)

Answer 1

FROM USPS.COM

First-Class Mail Prices

Quick, affordable service for your basic mailing needs.

    * The least expensive, most immediate option for mailing postcards, letters, and large envelopes 13 ounces or less

    * Prices based on shape and weight

Postcards - $0.27

Size limits:

    * Minimum: 3-1/2 inches high by 5 inches long by 0.007 inch thick

    * Maximum: 6 inches long by 4-1/4 inches high

    * Additional postage required for larger postcards

Bottom line:

Put a stamp on it and you'll be fine.

Answer 2

It depends how much it weighs.

Answer 3

Ship it where?  If it's within the US no, that's a pretty standard size envelope.  However the cost is not just determined by the size of the envelope but also the weight of the contents.  You should head down to your local Post Office and have then tell you the exact postage for what you want to mail.