Continuing from my above post, I have personally bought many products from US and China recently (to be shipped to me in the UK) and in every incidence, the seller has stated clearly on both the product page and at check-out that customs fees and taxes will be applied upon importation to the UK. They also offered a customs service (sometimes offered for free by the seller, sometimes paid for by me at check-out) to avoid any issues and to pre-pay all fees and taxes due.
In the one incidence that I was not informed of taxes and customs fees, and it was held at the border, the seller resolved this immediately and refunded to me all expense incurred.
FedEx have ‘foodstuffs’ on a list of prohibited items for import into Mexico, so the issue with Huel could be that it is food.
It sounds like from what @Carlos is saying, that Huel did not complete the paperwork correctly (or at all?).
It is very poor customer service indeed to simply ship a parcel to another country, without first checking the import export restrictions for that country, without informing the customer of all fees and taxes that will be incurred, without completing the neccessary paperwork, and then tell the customer that they are responsible for non-receipt of goods: “you haven’t received your order because you haven’t paid the tax required by customs”.
Customs services are available from almost every courier - this hasn’t been offered, and the paperwork was not completed. @Carlos is not at fault here and definitely should receive a refund.