The USPS is a well known and recognised USA mail carrier with a good reputation. When however, you use USPS to ship items outside of the USA, they then use local mail carriers to delivery to the final destination. In the case of shipping to the United Kingdom, USPS for some unknown reason have chosen to use Parcel Force to deliver to the final destination. There are however some problems with this...
1. Parcel Force will charge you £13 ($23) just to clear your item through customs, and that charge is passed on to the receipient, and not included in the USPS shipping charge.
2. Despite Charging you the £13 ($23) to clear your goods, Parcel Force have no interest in or accept any liability for failure to clear your item, or to clear it correctly.
3. They tell you about this charge by eventually sending out a standard INVOICE to your delivery address asking you to pay the value BEFORE they will deliver your item.
Anything shipped to the UK that is not a gift, documents, or books incurs VAT (Value Added Tax) at a rate of 17.5%. A good percentage of items also incur a DUTY of anything from 2% to 6% depending on the "description" of the goods on the shipping label.
For example, I recently bought an item from the USA for $2100. It was an electronic diagnostic tool for cars, boats, etc. The Seller wrote on the shipping ticket "CAR COMPUTER" and wrote the insured value of $2500 in the insured value box.
UK Customs, took the $2500 insured value, added the $130 shipping charge, then added another 4% import duty which is applicable on CAR PARTS, then added 20% of the total and then coverted it to £ giving me an additional charge of just over £300 ($540). The actual value was of course much less, but Parcel Force are just not interested, and you have no choice but to pay their charges to just not accept the goods.
In contrast, International Couriers such as DHL, UPS, EMS, etc DO seem to take responsibility for the goods right from getting the shipping ticket correct, through to ensuring that the goods get delivered on time.
So, in my personal experience, always use an international courier to ship to the UK if you want your item to arrive on time and without some crazy additional charges thrown in for good measure.