A signed document is one of the things you can use to defend yourself in the event of a chargeback. It's not a chargeback proof. Paypal is notorious for freezing people's accounts if they detect a very slim sign that people are doing a risky business with their accounts. Even if you manage to keep the chargeback rate low, you may still face possible problems with Paypal.
I think he/she is right concerning the issue. However, I must repeat paypal is very risky. Having too much chargebacks may lead to paypal account suspension.
I would not do it.
I recommend prepaid western union or money gram for a flat rate found at many general good stores. I believe those would be the best alternatives.