The same restriction applies to UK postal service. In Britain it isn't uncommon for a postal clerk to ask if the package contains jeweller or monetary devices (probably due to concern about theft/insurance etc it is restricted)
Having said that it isn't uncommon to send banknotes inside Christmas cards and coins ect I normally just describe as tokens and haven't had a problem.
I try not to buy from US sellers if possible because it is a considerable gamble at my end. While some vendors of notes on
ebay describe the contents of the package as "printed paper" others go all out describing it as a collectable and full value declaration as a sale. Sellers with experience realise these rules and several send bills standard delivery uninsured with no declaration (they take a risk that they don't go missing but must cover that risk in volume sold)
If the box "gift" is ticked this saves me 20% VAT if not I end up paying an admin surcharge of about £7 + 20% on the value declared. It means several bargain notes turned out to cost may well over the odds. The laws on seconhand goods in the country means I should not be liable to VAT anyway since on second-hand goods the VAT has already been paid...so I should only be liable to VAT on the vendors profit. But in the case of an obsolete currency it is only worth the value of the paper, any other value attached to it by a collector is just speculation so I don't understand the need to declare its full value in order to gift the government money for no reason.
Rant over.