On OpenSea, using Polygon Mainnet when purchasing an NFT the metamask popup shows only the gasFee in MATIC but not the listed price of NFT in total, why?

You are trying to purchase an NFT that is being listed in WETH instead of MATIC that is the reason they only see the transaction fee price in MATIC being charged to them instead of the listing price which is in WETH that is why the ERC20 token transfer (WETH) not being displayed on the MetaMask pop-up window. I understand that this may be confusing and I apologize for any inconvenience this may have caused.

I wanted to let you know that MetaMask only displays native chain tokens in the pop-up window. This means that tokens that are built on top of the Polygon blockchain, such as ERC20 tokens, will not be displayed. You can still send and receive these tokens, but the transfer will not be displayed in the pop-up window.

You can try again buying any NFT that is listed in MATIC, you will see the listing price is reflected on the metamask popup as matic is native to the chain.