Update invoices.md to account for currency conversion fees (#8611)

I faced a roughly 1.5% in conversion fee because I thought that I would
be paid the salary in my currency which is Canadian Dollar.

I got that impression from the existing description in this page. So now
I am editing the page to help other people to avoid facing this.

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Co-authored-by: arafatkatze <[email protected]>

content/benefits-pay-perks/pay-expenses/invoices.md

@@ -10,6 +10,11 @@
 ### How to Calculate the Invoice Amount
 
 - Invoices should be submitted in the currency used in your contract (and USD by default), if expenses were purchased in a different currency, please use this currency converter: https://www1.oanda.com/currency/converter/ and use the date of purchase. (Do not use the Veem currency converter.)
+- NOTE: If the the currency in your contract differs from your payment currency, Airbase convert your invoice amount into your desired payment currency. This may result in a favorable or unfavorable exchange rate. To mitigate this:
+
+  A. Retain a contract in the same currency as your payment currency. Updated employment contracts are handled by the People Team.
+  B. Maintain an account in the same currency as your contract to avoid exchange rate instability.
+
 - Your monthly invoice amount depends on whether you work a full month or a partial month.
   - **Full month**: to find your monthly invoice amount, refer to your contract, which should state the monthly rate (or take your annual pay and divide it by 12 to arrive at the monthly amount). Most months are full months, including when you take PTO or when there are holidays.
   - **Partial month**: use the formula below to prorate partial months. Partial months happen, for example, if your contract starts in the middle of a month.