On the face of it, it seems very simple for Accurascale to just implement a shipping rule to 'send all US orders valued between $335 and $800 via the mail and all others via courier until Aug 12'. However, it's not that simple at all.
For instance, let's suppose Accurascale implement the above shipping rule - but then US tariffs change again before Aug 12 in a way that makes shipping orders between $335 and $800 cheaper by courier instead of mail. What happens if Accurascale fail to spot those tariff changes and don't change their shipping rule accordingly - resulting in their customers missing out on the lower cost option. Would a US customer be entitled to be upset at Accurascale for imposing the higher cost option on them?
It seems obvious to me that to avoid the above scenario, Accurascale would have to commit resources (i.e time and money) to:
track US tariff developments on an ongoing basis for changes; and,
for each new change - investigate various scenarios involving shipping options, order values etc. - and then implement new rules to pick the best shipping option.
The safer (and cheaper*) option for Accurascale is to let their American customers do their own research and choose their own preferred shipping option - that way American customers only have themselves to blame if they have to pay higher than expected tariffs.
*I expect Accurascale have crunched the numbers when making their decision and have taken into account size of the US market, and the likely negative impact on sales etc.