Oh, it''s good to know the idiot who created our chicken tax has relatives in the UK ;-)
I think Decent's approach is sensible. It is a small firm, roughly three-dozen employees. Can you imagine the additional headcount, paperwork, etc to insure tax-compliance in every country they sell to, which is almost all of them -- 175 or so. And that's not a one-time process; one has to monitor and be liable for all on-going changes (for the USA this may be daily now). Much more sensible to leave it to the customer's discretion.
Note that with taraiff-du-jour madness here, for a time firms such as DHL would no longer deliver to the USA. The compliance cost and complexity were well beyond them.
The Apple AppStore can and does charge tax as they have the scale to do so. They are inherently global with tax staff for every nation already, and they have billions of annual transactions to absorb these costs.
I do not see how any small firm such as ACS for example could do this. For small firms with product lines that cross tax brackets, putting the burden of honesty on the customer is the only system that works. If a customer chooses to do otherwise, they are assuming the risks.