literally from the article: "The practical effect of the exit of the U.S. would be a rate increase of at least 300 percent on postal parcel traffic to the U.S. from heavy net exporting countries as rates kept artificially low for decades begin to normalize" 3x increase in costs for shipping sounds pretty disadvantageous. It's not like the US is producing a lot of stuff being shipped international via post.
That quote from the article is about "parcel traffic to the US" not from the US. The next sentence is the relevant one to your argument, I think: "U.S.-based international shippers will also pay more, at least over the short-term, because USPS will cancel negotiated service agreements (NSA) covering international shipments if the withdrawal takes place, White said."
But how many US manufacturers are there that ship internationally via USPS?
Our company is one. About 20% of our sales are outside the US. We ship international shipments via USPS exclusively because it has always been the best value.
If they sell to the public, probably quite a few. When the sender uses a parcel service the recipient gets to pay the fee for customs brokerage, but with the post it's included.
Commercial recipients know about brokerage fees and can estimate how much the surcharge will be, but Joe Random has no idea and will complain.
There is a market where people buy from us based companies, the items are made elsewhere but people are more comfortable buying from a us brand
Won't this increase the price of goods imported into the US?
Practical upshot presumably being the end user will pay more for shipping instead of it being subsidized at the tax level by everyone. For better or worse.
The USPS isn't subsidized by taxes, though - instead the subsidy has been coming from domestic package shipments, which should become cheaper now that they aren't being relied on for that.