no condoning it, but if the seller was insuring it and it actually sold for $2000, $20 postage wouldn't actually even cover the full value. Extra Cover is $1.50 per $100 or part there of, so:
Actual Postage - $2 (for a padded envelope) + envelope cost
Registered Label - $3.80 (I think)
Extra Cover for additional $1900 (registered covers first $100) = $1.50 x 19 = $28.50
Total Postage = $34.30
Plus you also pay ebay fees on postage these days, so you can add another 10% to that.
Like I said, not condoning charging $20 postage when you are going to send in a white enveolpe, but postage costs these days are becoming ridiculous. I've recently had to change my international postage to minimum of $26 as the cheapest way to send international with tracking costs around $25 to US and $28 to the UK (excluding any packaging and envelope costs) and I just have had too many things go missing overseas to send without tracking.