"Pay what you think it's worth" might be a good way to mentally offset this? That way, you're not affixing a price to your work but instead just asking for a demonstration of what they value it at. It's probably a unilateral upside if the entirety of your payment process is 'post me a cheque if you feel it's worth it!', for the following reasons:
1) By not affixing a price to it, people will pay what they think it's worth. By affixing, say, $20, those who think it's worth less may not pay but those who think it's worth more won't pay more. There's a small risk that people may stretch to make your price tag but instead not bother but my completely untrained gut would say that'd be minimal?
2) You're already relying on people to go out of their way to send it based on goodwill. You're not going to lose customers simply by floating the amount they can choose to send.
3) Your conscience doesn't tell you 'hey, are you SURE this is worth $20? What if something breaks for someone? You forced them to pay $20 rather than $5, you owe them more than this!'. That's got to be liberating.
Of course, PWYW works mainly if it's a supplemental income and you're not relying on it to feed yourself :) That said, I'd expect it's pretty confidence-boosting to see someone voluntarily pay for something you made, since it basically says 'hey, I think you're worth this, thank you!'
The OP and this discussion is incredibly inspiring too, it makes me want to build and release stuff in my spare time that people are completely free to pay as little or as much as they want for, just to see what happens :)