Like @anon52464727 said:
This is what Tuta officially said under “How can I upgrade my Tuta account with cryptocurrency?”:
[Tuta Support | Payment | Tuta]
We plan to add Bitcoin as a payment method to Tuta in the future. You can already buy Tuta gift cards with the cryptocurrencies Monero or Bitcoin or with cash via our partner Proxystore.
To redeem a gift card from Proxystore, please follow the instructions in Proxystore right after ordering the gift card. You will get a gift card link. When you use the link, you need to choose ‘Use existing account’ or create a new account, depending on where you want to apply the credit to. Once you have added the gift card, you can check the credit on your account under Settings → Payment. When upgrading your account, you can now choose ‘Account balance’ so that no payment details need to be linked to your Tuta account.
If you require further assistance, the Tuta Support link provided above offers more detailed information that should hopefully assist you in finding the answers you seek.