Is there any way to enter all my sales prices in Canadien currency, and then have the program multiply those amounts by the exchange rate and have the invoices in MXN?

Welcome to the Community.

Let me share some information about using multicurrency in QuickBooks Online.

Yes, you can use multicurrency in QuickBooks Online. To start with, follow the steps below to enable the feature.

Select your home currency and enable Multicurrency. The home currency is typically the currency your business employs for its transactions based on its location. Income and expense accounts always utilize your home currency.

When you configure QuickBooks, it automatically establishes your home currency. If necessary, you can modify your home currency.

Once Multicurrency is enabled, you cannot disable it or alter your home currency. This is due to the fact that the currency conversion data will need to be consistently accounted for in the system moving forward. If you are uncertain, we advise consulting with an accountant.

You can read this article to learn more about using multicurrency: Set up and use Multicurrency in QuickBooks Online.

If your home currency is Canadian dollar, you can set your customer currency as MXN so you can invoice them in foreign currency. This way, even if you can't enter the sales price to Canadian dollar, you can still track the total in Canadian dollar and MXN within the invoice.

The exchange rates will still be calculated but the invoice due will be in MXN once you open the invoice. You will still be able to see both currencies in the invoice.

For future reference, I'm adding this article about multicurrency common questions: Multi-currency FAQ.

Let me know if you have questions about using multicurrency in QuickBooks Online. I'm always here to help. Have a great day.

@puertasportalmex 

You can prepare your Invoices data with the following template

Then use an importer tool to proceed

https://get.transactionpro.com/qbo