Generally speaking, anything under 16 oz should use First Class Mail and anything over 16 oz should ship with Priority Mail. There are a few exceptions, of course.

What is the difference between USPS first class and priority mail?

The main difference is in the size of the packages that can be delivered via each service. First class mail is limited to smaller packages that weigh less than 3.5 ounces, while priority mail can include packages that weigh up to 70 lbs. ... First class mail is considered a priority mail over other standard mail services.

Can you ship first class in priority?

You can send standard postcards, letters and large envelopes and small packages up to 15.99 ounces using First Class Mail. If your First Class package is over 15.99 ounces, you must upgrade the mail class to Priority Mail.