Add "Mispayment Buffer"

[JustinDrake]

Several OpenBazaar vendors I have talked to mentioned they had problems with buyers paying most of the quoted amount, but missing just a bit to complete the transaction. This could happen if:

• The buyer's wallet subtracts the mining fee from the sending amount
• The buyer's wallet rounds down the amount paid
• The buyer uses an exchange, which may deduct a fee from the withdrawn amount

A solution for this problem is to allow for an optional "Mispayment buffer" like what Coinbase does:

Mispayments created unsatisfying customer experiences and a lot of work for everyone involved. We’ve built a way to allow merchants, through our v2 API, to create simple mispayment buffers. Mispayment buffers are percentages that can be applied to either side of the quoted order price, declaring a customized, merchant-specific range of acceptable payment amounts.

For example, let’s consider an Order for 10 BTC with a 1 % underpayment buffer. A customer who accidentally pays 9.9 BTC will successfully complete the order, while someone who pays 9.8 BTC will not. This gives merchants more control, allowing them to determine what is best for their business.

[cpacia]

This would probably need to be added to the server side as well. My only problem with it is buyer know they can always pay 1% less than the price. Other than that it would probably solve some problems with orders being rejected.

[Dekker3D]

Maybe make the buffer user-settable, so people won't be sure what the level is and might end up spending more fees to get it to go through if they try to game it.

[JustinDrake]

In practice a merchant that feels the mispayment buffer is abused can simply refund the buyer

[jjeffryes]

@cpacia @tyler-smith are mispayments still a concern? We haven't had reports of them for quite a while.