Currently all SupaDupa boutiques operate in a single currency (see: List of currencies supported). If you have the requirement to sell in different currencies then the way to approach that right now is to setup separate boutiques for each currency you wish to sell in and have a different country-level domain name for each one.
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For example, if you wanted to sell in GBP and USD, you could open two boutiques, one with a ".com" domain, and the other with a ".co.uk" domain. Your marketing materials in each country could mention the relevant domain for the target audience.
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When doing this you will need to add your products to each boutique and keep changes in sync between the two boutiques.
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It is not currently possible to offer a currency conversion between different currencies on the site, and it is not currently possible to link the two boutiques together.