Why walmart.com charges credit cards in portions?

It sounds like they're getting a pre-authorization, also called a "hold". The bank confirms that the money is available and sets it aside, but the merchant has not laid final claim to it.

Holds expire if they're not finalized within a certain period of time. The amount of time depends on your bank. While a hold is still valid, a business can use the authorization number to complete the charge. They can also create an entirely new charge and let the previously authorized charge expire on its own.

This is common practice for things that are reserved, like hotel rooms and rental cars. People don't usually notice it unless they have very tight funds.

When you place the order, they check to see whether the card is good, but they leave the the money on the card until it ships. Then they take money when it ships.

They place a hold to see if the card is good. The hold is reversed when the actual charge is approved. Walmart is not the only one. Gas stations do this often. I'm now getting my groceries delivered thru Instacart. As soon as I place my order, the estimated charge is made, including the items, service and delivery fees and tip. Once the order is delivered, I can change the tip amount, and out of stock items are refunded, and I am only charged the actual amount. The initial charge disappears.

It might be something to do with your bank.

Best Buy does the same thing. It’s their way to reserve the money before it actually ships. It’s also to prevent fraud. If you didn’t place the order and if you have auto notification set up, you’ll get a notice that your card was charged so you can dispute it.