When importing the ‘receiving carrier’ normally handles the package through customs.
This means Royal Mail, FedEx etc may charge you.
Charges are mostly based on the declared valuation from the sender, but they can be reassessed based on a perceived value in the UK
(From the US they should be more likely to use the declared valuation, whereas a number of Hong Kong / Asian sources are known for false declarations)
For a t shirt I would not expect import duty, but import VAT at 20% might be chargeable
Any charges could be based on both item value abs postage costs - so don’t assume that any shirt will be below the VAT threshold
If it’s considered to be a designer t shirt then the valuation may be increased for the UK
If there is import VAT due then the carrier handles it and charges a fee
Fees can vary between carriers and I’ve seen between £8 and £20
If there are no customs charges then there is no handling fee
Assume that there is something to pay, and count a win if you don’t have to
If there are charges then you will either get a card in the post or notified by email / text.
If you keep an eye on the tracking reference then you can often track its status at customs and may be able to pay it without waiting for a card to arrive
It may be held centrally until paid (use your card to pay via an online form) or at your local distribution office (in which case you can pay online or turn up to pay and collect)