Hello there, Bev14a.
QuickBooks Online (QBO) uses the default R symbol to the invoice transaction to represent the ZAR (South African Rand) currency. Thus, making it print as ZAR instead of R is not possible.
It would be nice to be able to expand the customization option to invoice transactions. Here at Intuit, we always aim to meet the demands of our customers.
While we don't have an exact time frame as to when will this be available, I encourage you to visit our QuickBooks blog. From there we share the latest QuickBooks news and feature updates of the product.
You might want to visit the following link to become more familiar with the features and functions of QBO:
Be sure to drop me a reply if you have additional questions. I'd be more than happy to help. Stay safe and have a good one.