150 Postage definitely isn't impossible, especially with upgraded envelopes and expedited shipping, it's more that it runs on the "premium currency" which might make it difficult for folks to get to that level of gameplay.
Ultimately, if the goal is to increase spending on usps postage, everything is kind of working as intended and this isn't necessarily a problem. But free-to-play options to accumulate Postage would make the higher level upgrades more reachable and let people budget a bit more while still seeing all the content.