How would Amazon track a voucher? It’s a physical scratch off code, sealed by Mullvad before they send it over to Amazon. More importantly, if you think that was possible why would Mullvad be unaware of it and/or lie about it? Just go with the vouchers if you want untraceability. They’re also cheaper in USD than other methods IIRC, at $29/6 months and $57/12 months.