how large first class packages
Large Envelopes Large envelopes must be rectangular to qualify for flat pricing. Large envelopes that exceed size requirements of a letter will be charged package prices. Packages Packages can be sent using Priority Mail ExpressÂ®, Priority MailÂ®, First-Class MailÂ® Parcels, USPS Retail Groundâ¢, and Package Services. Contents must not weigh more than 70 lbs. Lower weight limits may apply. Girth is the distance around the thickest part of the package.