wool soap

interesting to see companies advertising products on mail such as here on this 1921 envelope from a Chicago company (see extra for name) to Norway...I guess wool soap may still be used in some form?

this is one such example in my collection of underpaid mail to Norway from around the world....the study interest is the amount the recipient was charged according to the rules of the UPU and Norway Post's use of their special "postage due" stamps correctly stuck on the back of the envelope to denote payment made