I send first class packages with stamps all of the time and use Label 400 on the package. Label 400 is the alternative to digital tracking number and is free with first class package and higher rate classes when using stamps. There is still the 10 ounce rule on packages with stamps (formally 13 ounce uni-bomber rule) and now packages which are thicker than half an inch with stamps must be handed to a clerk (also adopted as a result of the bombs in small packages in the Austin area a couple of years ago).
Announcement of latest change:https://about.usps.com/newsroom/nat...is-month.htm
Tracking label order form which has actual bar codes not blank in the center as the image shows:https://store.usps.com/store/produc...0-P_LABEL400