I ws just assuming it was a wrapper for a policy document. I guess I am trying to determine if the deficiency is based on it being a parcel or something posted at the printed matter rate.
It was common practice* during the 1920s/60s period for mail with a postage deficiency addressed to large businesses to be placed on a single (daily) docket with the item 'on top' having the full amount in postage dues affixed when the addressee (or their agent) paid up. This is only one explanation for postage dues being used to pay larger amounts of deficient postage of course and your item may certainly have been from a taxed parcel however I believe it is important to explore all possibilities before searching for a specific type of mail article.
P.S. this practice occurred at the Hobart GPO and Launceston post office at least.
P.P.S. does this item have part of a Tasmanian postmark under the p/dues? The Australian Provincial Assurance Association did have an office in Hobart during the 1920s/30s period (trying to find the "Tasmanian Connection" re your query)