This is why Paypal's free options aren't always the best... You can actually do this with a real merchant account. Merchant accounts and a payment gateway come with a Virtual Terminal that allow Post Authorizations in most cases. So (like a hotel), you can actually authorize the card for one amount, then complete the sale for a higher amount. As long as the final amount isn't 20% higher then the first amount you're in the clear to my knowledge (but check with your merchant account provider). But this is definitely doable with a real merchant account and payment gateway that includes a virtual terminal for manual entry - it is called pre-authorization and post authorzation (or force post).
But yes your MERCHANT will have to have this account in their name.
As for charging in someone else's name you can do this if you operate as a payment aggregator - which will require you open a high risk account initially - the percentages are higher for this as a payment aggregator and it's harder to be approved as one because you are being trusted to essentially become your own Paypal. It is doable however as I have set up an aggregator recently for a merchant in an unrelated business.