I'm not a logistics expert, but here's what some rapid googling suggests is going on to me: DHL is indeed a subsidiary of Deutsche Post. It provides relatively cheap e-commerce services and a much, much more expensive courier service. The DHL e-commerce services have contracted with the USPS for many years to provide exclusive delivery of the "last mile" for many and perhaps all zip codes.
One website states:
DHL Ecommerce is an economical way to ship your parcels to their destination. DHL Ecommerce arranges for your parcel to be delivered to the destination country via DHLs logistics network and final mile delivery is done by the local postal service in the destination country.
That's why it's so much cheaper; DHL has estimated that over half of the total delivery cost is for this "last mile", which is the whole reason they outsource it to the USPS (which has already paid for the infrastructure to visit every house six days a week anyway) for their ecommerce products. If you want a clear comparison of how much more you'd spend for true courier services you have only to look at the shipping page on Estervals website:
Estervals Pipe House - https://www.tecon-gmbh.de/shipping.php. It's clearly an option, and I would bet both quicker and more secure...if you want to pay for it.
What does all that mean? I think it means that there's essentially no way to avoid the USPS for DHL e-commerce shipments from overseas, and almost no way to avoid DHL e-commerce for purchases from Germany.
Unless you're willing to pay 119 euros and up for true courier service (i.e. end-to-end delivery). I suppose you could get competing quotes from Fedex and/or UPS for comparable courier type services, but a) I'm pretty sure they'd cost about the same amount, and b) your vendor might not offer them as an option. Bear in mind retailers (and wholesalers too) typically pick a single delivery service to work with to simplify their internal processes and to maximize their negotiating leverage for better rates.
Do other people think the handoff to a local postal service sucks? Apparently so, and in many countries. This website shows almost 20,000 comments, with the aggregate overwhelmingly negative:
DHL eCOMMERCE TRACKING | Parcel Monitor - https://www.parcelmonitor.com/track-dhl-ecommerce/
I think what this adds up to is that your experience will vary depending on the specific parts of the USPS that your particular your package happens to come in contact with. And there's not a damn thing you, or Estervals, can do about it except a) not order from them (or probably anyone else overseas), or b) pay up for courier service. I suppose Estervals could explain all this (assuming I've gotten it right) on their website, and make it crystal clear that people using the lower German Post WorldNet DHL rate do so entirely at their own risk. But given the large cost differential my guess is customers would still order using the much cheaper DHL e-commerce rate, and still be surprised and angry if the package doesn't show up.
If all of the above is right I don't see what Estervals can do. They can't possibly influence either DHL or the USPS to behave differently in any way. In theory they could withdraw the cheaper rate, but I suspect most people would rather take the chance of an unsuccessful delivery (which I would guess is well under 10%) over the certainty of paying triple the shipping cost. Or Estervals could refuse sales that will wind up in the hands of the USPS. Or they could estimate the extra cost of indemnifying customers and raise prices by an amount to recover that loss. This last is a real pain in the ass; the accounting is an extra burden, and there's extra work in ensuring that you're not being screwed by dishonest customers, who, sadly, exist. I think it comes down to what I said before. If this business is important to Estervals they'll figure it out. If it's not, they'll either ignore complaints or stop shipping to the States.