If you have products configured with the "Let buyers edit price" setting enabled, you can set each name-your-price product with a minimum price that the buyer cannot undercut (if they try, it just gets set to your minimum price).
We do not issue buyers a download link if they tamper with the price during checkout. In fact, when PayPal sends us IPN confirming payment, we check the price paid against the current price of the item (accounting for any of your discount codes they may have applied in the cart), and if they paid less than that, we send you a Fraud Notification instead of issuing them a download link -- sometimes that can happen inadvertently when a Merchant edits a product price while the buyer has taken an order for that product to checkout at the same time, so our notification gives you a chance to consider the matter and still process the sale manually if you wish.
As to the matter of buyers getting refunds from PayPal, that's pretty much just a "cost of doing business" for digital goods with PayPal, as their Resolutions evaluation tends to give buyers the benefit of every doubt when they claim they did not receive a valid download or that the download was not as-advertised. We can only track the number of times they attempted clicking the download link, but there's no technical way possible that we can prove conclusively that they obtained a complete, working, openable, non-corrupt file -- nevermind the question of whether the file actually is or does what they expected -- so PayPal just takes their word for it if they claim otherwise.
