There is a £2 monthly fee for the Euro account charged as a monthly subscription that can be cancelled any time. Inbound and outbound SEPA transfers are free from the Euro account. The USD account costs £5 per month and can also be cancelled any time. You can send and receive US dollar transactions for free.
You can also transfer money between your Starling Euro, US and GBP business accounts, which involves a currency conversion fee of 0.4% on top of the real exchange rate. If you regularly receive euro transactions from clients, you can therefore save a lot of money by adding a Euro account to receive free transfers. If you need to move that money to your Business (GBP) account, it is cheaper with the 0.4% currency conversion fee between Starling accounts than to receive euros directly from clients into the Business account which would cost 2%.
The free SEPA transfers with the Euro account and free USD transactions with the US account are really useful for those with clients paying in these currencies. The accounts can also save you money if you travel regularly in Europe or the US and want to avoid the risk of merchants applying their own fees to currency conversions. Starling does not apply a fee when you use the debit card in another currency, but the merchant may use their own exchange rate with additional fees if you choose to pay in the local currency.
The Euro and US accounts do not come with a separate debit card. Instead, the accounts are attached to your Business bank account, even though the different currency balances are separated in the app.
To use the euro or dollar balance with the Business Mastercard, you have to switch on the relevant currency in the card management section of the app. The card will then automatically use e.g. the euro balance for euro transactions and pound balance for pound transactions.