Mullvad offers flat rate $5 (no matter 1 month or 12 months or 120 months) and never have any sales so I'm surprised to see these[1] prepaid amazon cards ARE offering discounts: 12mo @ $4.75/mo & 6mo @ $4.83/mo esp. when these are /physical/ code-card purchases
They do provide a nominal discount if you pay with Bitcoin, though I assumed that had something to do with the lack of payment processor fees so it doesn't necessarily explain why the Amazon prices are different.
They don't have any other rate for you and me as individuals, but I'm sure they offer them slightly cheaper for resellers like Mozilla and Malwarebytes.
Those resellers then charge about the same price as Mullvad, but get to keep a piece of that as profit.
[1] https://www.amazon.com/Mullvad-VPN-Devices-Protect-Security/...