You should be as accurate as possible when entering your dimensions/choosing your package type, as this can greatly affect the price you seen in Shippo.
For USPS, if you're selecting a package type from the drop-down menu, instead of entering your own dimensions, you will only see Priority Mail and Priority Mail Express prices.
This is because all the items on the drop-down menu of package types have some kind of special price associated with them (e.g., Soft Pack, Flat Rate, or Regional Rate). If you want to see a price for a Flat Rate envelope or mailer, you would choose the appropriate one from that list.
For all other packaging, even if it's branded by the carrier (for example, a mailer or box with a USPS logo on it that does not say "Flat Rate" or "Regional Rate"), since there are no specific rates associated with that packaging, it will not be listed on the drop-down menu.
For these packaging types, enter the dimensions of the mailer and see the correct rates for it. For envelopes, make sure to measure the thickness or "height" of the envelope to get accurate rates - you can put in fractions of an inch as 0.25 or 0.5 inches, for example.