US territories and possessions should be guaranteed one seat in the House of Representatives for every twice the average number of people represented by each Representative of the 50 states. For example, if 600,000 is the number of people represented by the average member of the House of Representatives, then, for a US territory or possession to be allowed a representative, it would need to have a population that rounds out to 1.2 million (2 x 600,000) instead of rounding to zero. In other words, territories and possessions would be entitled to one representative for every 1.2 million people. Thus, they would be required to have a population between 600,000 and 1.8 million to get one representative. To get two representatives, their population would need to average to 2.4 million, or fall between 1.8 and 3.0 million people, and so on.
Under this formula, only Puerto Rico would be eligible to elect a Representative, in fact, it would be entitled to three Representatives because it has a population of nearly 4 million. All other territories and possessions would not be eligible because of insufficient populations.
Because these territories and possessions are not fully permanent members of the US, they should not be entitled to full representation. But because they are part of the US, they should be entitled to some representation. Half as much representation compared to the regular US population seems fair.
However, because these areas are part of the US, and because these territories do receive half as much representation (regardless of whether or not they actually have Representatives due to rounding), all territories and possessions should be subject to half the federal tax rates charged in the US. For example, if a federal tax on something is 2% in the mainland US, then these territories and possessions would be levied a 1% tax.