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.