Many of those listed are UK companies.
When buying from the US you will usually be limited to the shipping services offered by the seller. Many have contracts with one or two companies and will only use those companies.
The usual options on parcels are the US Postal Service (USPS), UPS, FedEx and DHL. That is pretty much it. And even then, if the company has a deal with UPS, they may not bother with DHL, even if you request it.
The company I did most of my business with had no options. They had 4 methods - USPS to post office boxes only. UPS for other ground deliveries, DHL for Rush or 2nd day and USPS Priority for Alaska, Hawaii and US possessions. Period. Take it or go buy somewhere else.
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