Can I combine two gift cards to have the mixed amount?

Hi there,

when you redeem a currency gift card it will be added on top of your current balance, so if you currently have 0 adding 20 will result in 20 and adding the 50 will result in the total of 70. So that should work just fine. Assuming it's all done in the same
currency and region.

Also keep in mind that xbox 360 content can only be purchased via the account currency on an actual Xbox 360 console and you should always use your own region and currency to purchase content.

You can apply both gift cards to your account and they will combine as 1 Microsoft account credit. For example if you wanted to buy a game worth $100 but you had 2x $50 gift cards, you can apply both of the $50 gift cards and then purchase the game (as they will combine both gift cards into a single $100 account credit)

To apply gift cards to your account use this link: https://redeem.microsoft.com/

Alternatively, you can also apply gift cards to your account by opening up the Microsoft store app > Hovering over the menu > then select “redeem”.

Hello!

How much in VBucks did you purchase? If the amount exceeded the gift card (don't forget sales tax), it would have charged the credit card after the gift card amount was used up.

You've kind of combined two different things. Did you use Microsoft Rewards points for an Xbox gift card or did you redeem gems from the Xbox Rewards program for rewards credits?

If the refund is showing on http://commerce.microsoft.com, that means you had that currency added to your account.

I used my gems for gift cards

you should not need to combine them. Choose one gift card as a form of payment which will reduce the total owed then choose the second gift card.