I like your idea about entering product cost into the Product information field as USD. However, we're unable to enter it, even if the multi-currency is turned on.
The currency appears in the Payment, Deposit, and Balance Due columns on bank and credit account histories as well as in the purchase forms. What we can do is enter the item/product in QuickBooks transactions. But, before that, we'll need to ensure that the multi-currency feature is turned on.
Then, assign the supplier's name to US currency. After we create a supplier transaction in a foreign currency, QBO creates an AP account in that foreign currency. Here's how:
Go to Expense on the left menu.
In the Suppliers tab, click the New supplier button at the upper-right corner.
Enter the needed information.
Then, select USD United States Dollar currency for this supplier.
Then, Save.
By the time we create a transaction with the items to this supplier, QuickBooks automatically calculates the rates. To view the same thing, go Chart of Accounts and open the account where transactions are deposited. Please see the sample screenshot:
As additional option, explore opening a multi currency business account at TW as the virtual bank and utilize the bank feed. I noticed their service is available in South Africa.