The keyword here is “autosubscribe” or at least its presence or absence. And your awarenes of this.
You may think you are having JUST one transaction charged to your account. But this this may not be true at all.
Your transaction may be treated as a subscription, which automatically gets renewed  and possibly upgraded without notification.
And worse than that the vendor could increase the price quite substantially when charging your account for a following period.
The problem here is that PayPal does not regard a price increase as an unauthorised charge. Since vebndors do not make unauthorised charges.

The only unauthorised charge PayPal recognises is when a “third party” gains access to your account. By definition in the PayPal world all charges made by vendors are authorised. In my case I thought  I was purchasing 12 months of brain training exercises. The “basic exercises”! but a year later I was charged for premium exercises at double the initial rate for the basic exercises, namely Dollars 84 instead of Dollars 42.  The company refused to refund the charge, saying I had accepted their terms and conditions. And PayPal refused to refund the charge as it was authorised since it was made by the vendor.

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