Sorry, I've been referring to StS as the supplier and ACPT as the manufacturer. If StS have taken funds for the full amount then payment has been made.
As Merli stated, a written quote even via email constitutes a contract. If full payment has been made to StS as the supplier for this group buy, the terms of the contract are binding.
To quote the Fair Trading Act 1989:
Section 46 (1)
A person shall not, in trade or commerce, advertise goods or services for supply at a specified price if there are reasonable grounds, of which the person is aware, or ought reasonably to be aware, for believing that the person will not be able to offer for supply those goods or services at that price for a period that is, and in quantities that are, reasonable having regard to the nature of the market in which the person carries on business and the nature of the advertisement.
If StS knew they could not supply for the quoted price if there were changes in currency exchange rates the offer should not have been made. Having dealt with ACPT before it is reasonable to expect that StS should have been aware that prices from ACPT are subject to shifting exchange rates. As such the price should have been quoted in USD or the quote given as an estimate subject to the exchange rate at the time of payment.