You may also be asked to "confirm" the card and this involves a fee being charged to it and then you entering numbers off of that line of your statement (online statement is obviously the fastest way to get this number.) While this money is nominally still yours, there will be limits on what you can actually apply it to - i.e. you can't get it back and you can't convert it to another currency.
If, like myself for example, you had once entered a card several years ago into a paypal account that you have not used since that day you may find that you would no longer be able to use that card for any vendors who utilize PayPal even if you were to select the option of not logging in. This is what has happened in my case and upon attempting to donate to a charity I found PayPal refusing to allow my donation unless I logged in to my long-forgotten account with them. They then demanded I provide additional personal information and either a land-line phone number (which I stopped using about 2 years ago) or copies of a passport. In addition they also required I pay to have my card verified before the donation could be processed - which will now take a few days. In lieu of that I could use an e-check, but at the dollar ammounts being donated the fees incurred upon the recipient would eat up most of the donation. Also, since they're already charging me the "confirmation" fee this means that PayPal fees would represent most of what should have been a small but quick and easy donation.
Next time I will be driving in in person and handing them cash. Paypal has made electronic payment so annoying and exploitative that it can, in some less common cases, completely negate the advantages and convenience of doing things online. Congratulations PayPal - you have become such a parasitic middleman that forgoing the advantages of technology has become preferable to putting up with you.
