I'm afraid you can't. Even if you buy NFTizer Flex Contract without the Standard Version, you will not be able to login.
Here is why:
1. NFT Collection Maker 2.0 Standard is the main product which is either a Monthly, Yearly or One Time Subscription.
2. NFTizer Flex Contract is an Upgrade or Up-sell which adds the Flex Contract features in your NFT Collection Maker 2.0 account.
3. Since NFTizer Flex Contract is not independent, you will need to subscribe for the main product NFT Collection Maker 2.0 Monthly, Yearly or One Time subscription first. Else you can't login with only NFTizer Flex Contract in your account.