Are there any withholdings Taxes that foreign / overseas clients would need to consider? 

Are there any withholdings Taxes that foreign / overseas clients would need to consider? 

There is a lot of information available. Please see the following link for more details: https://learn.microsoft.com/en-us/partner-center/marketplace/tax-details-marketplace 

There is a lot to consider, especially with overseas clients and Microsoft-managed countries.