http://www.paypal.com/cgi-bin/webscr?cmd=p/gen/ua/policy_pbp-outsideThose are the rules for PayPal's buyer protection --- READ IT! It is in your best interest to know what PayPal will and will not provide coverage for - know it to protect yourself when buying and also how to protect yourself when selling!
You may be able to argue that under the "Claims" section (sec 4,a, 2, iii) that "The description could have been reasonably misinterpreted by the buyer or the seller. For example, if the item is a different color than advertised (e.g. the item is aqua-marine but was advertised as teal);" - your definition of "light 15lb" may be different than the buyer's definition.
You could also argue that the buyer has significantly altered the item (by having it plugged and re-drilled) and the item is now worth even less than originally sold (at least worth less to you). The buyer should have contacted your before making any alterations to the item - the item is no longer in the condition it was received.
You could also just offer to refund partial money to the buyer - if they accept the situation is considered resolved. If you are that worried about it, refund the shipping - maybe the guy will be happy to get $15 back.
IMO, if the buyer was worried about the weight he/she should have had the ball weighed before it was plugged and re-drilled.
Look at it this way - if PayPal determines the other guy is entitled to a refund, he will have to pay to ship the ball back to you. When you receive it you can complain that the ball is not in the same condition as when you shipped. Also, the guy spent $45 on the ball, another $40-50 for the plug and re-drill, and add another $10-15 for shipping it back to you. The most he will get refunded is $45, so he will be in the hole for $55-60 (I don't even know if PayPal will refund your shipping costs???). That's something to think about. Tell the guy to ship the ball back and refund his money - he will be out $50-60 with nothing to show for it.
