Currently, there isn't a way to get rid of the "A" in front of the total invoice amount. It's because the program uses A$ since it refers to the Australian Dollar. The screenshot below serves as your visual guide.
I recommend looking for a third-party app that offers a platform of having no "A" beside the total invoice amount. Please choose an application that can integrate with the system by contacting the app's support team.
Removing the "A" beside the total invoice amount is a great idea. Please know that your voice matters to us as it tailors to your business needs. With this, I'll take note of your suggestion and submit this to our product engineers. They can evaluate your preference for future releases.
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When your customer pays the invoice, you can record the payment in the software. It ensures your customer balances are up to date. Just go the + New button from the left menu. Then, select Receive payment as seen in the screenshot below. To learn more about this process, check out this article: Record Invoice Payments.
I'm always here to help if there's anything else you need. Take care always, @tonyperinotpe.