When you are sending within the USA then you have 4 options that you can choose from, they are as follows
Priority Mail Express - "next-day to 2-day guarantee by 6 pm", this is their fastest domestic shipping service, they deliver 7 days a week, 365 days a year (with very limited exceptions), with next day delivery available on most U.S. Address with a money-back guarantee if it is delivered over the time specified. Prices start at $26.60.
Priority Mail - This is the best-selling domestic delivery option and has a delivery time of 1-3 days depending on where your package starts and where it is going. Tracking and insurance are included in this package with shipping boxes and envelopes being free, with Priority Mail Flat Rate you don't need to weigh shipments 70lbs and under. Prices start at $7.95.
First-Class mail - This is the most affordable mail service for standard-sized envelopes weighing up to 3.5oz and large envelopes and small packages weighing 13oz. Prices start at $0.58.
USPS Retail Ground - is a reliable and economical way to ship items that are not in an urgent rush to get to the destination along with oversized parcels. With Retail Ground, you can still track with Ship24, but you will only be able to send your package via a USPS Post Office. The package can be up to 70lbs and 130 inches in length and girth combined.