Is it possible to use more than one tender when paying online for something?

You could if the first was a store gift card. Otherwise no.

Consider trying this - "sell" your gift cards to someone who is willing to give you the full value of the cards then use the money to get ONE card with the total amount on it so you can use it on the website you need for the item you want.  You might have a friend or relative that could do this for you.  (they can then use the cards anyplace else.)

Most websites don't allow multiple payment methods so it is very hard to find sites that let you enter more than one gift card if the card isn't THEIR card.

It's really better in a retail store where stores can handle the partial payments easier. Some may want a fee. In general, gift cards cannot be used to buy other gift cards.

Maybe you have a friend or relative using cash and can apply your cards for cash.

I did read that Amazon.com sells Amazon egift cards in any value, and they can be sent to yourself each one bought by a VISA gift card. Then, they can be deposited as an Amazon credit and used for a purchase from Amazon.com as seller.

This is not a sure thing, but I read it works.

Some stores like Amazon allow the purchase of their gift card with a credit card. I have the cards electronically sent to me and then add it to the Amazon credit.

Not if it’s a visa gift card. If it’s a gift card from the online store then yes. Best Buy let’s you enter several Best Buy gift card numbers. I believe the same goes for amazon.