Yes sadly I saw that too. I sent PirateShip support an email to see if there is any options beyond CVS.
However, PirateShip isn't the only one offering this type of service. It is similar to stamps.com, shiprush, goshippo.com, easyship.com, and multiorders.com.
They all leverage a USPS Commercial pricing contract to let you have access to the discount pricing as long as you use their service to order shipping/lable for USPS (and UPS/others too). Some pass along more savings than others but the business model is the same.
It allows small businesses to get access to lower rates without having/meeting the USPS commercial prizing quotas (which are not exactly small).
I saw the CVS sheets PirateShip with the rates too. I imagine it is possible to set up a set of zones to match it using the Geo Location plugin (zones are based on how far from the base zip code) but that seems like a lot of work to set up/maintain.
This client is more concerned about passing along the discounts to their customer than lables/orders so that their shipping rates don't drive away sales.
As an alternative, I told my client they could also apply a negative % discount to via the HikaShop's USPS plugin. The problem is the discount rates vary from 25% to 50% depending on zone & weight.
I'm not sure PirateShip is the best of these services; but those that have been in business longer like stamps.com do have an API
developer.stamps.com/
, but it is more than just commercial pricing. I've had one other client express interest in such labeling/discount rates/tracking services, but maybe others have here too. Not sure which one is the best one, but I believe stamps.com is the oldest and most established one so they may be a better choice to work with.