...
The bottom line: the customer sent you BTC with a low fee. That's their fault; don't bend to their demands.
Hypothetical situation:
A customer creates a server, and installs a Bitcoin Node. Anyone can do this.
The customer sends BTC to the Bitcoin Node that he controls to begin the scam.
The customer initiates a SELL at your BATM3 with an unreasonably low fee sent by the Bitcoin Node that is under his control. It shows as "unconfirmed".
After a day or two, it still hasn't cleared (of course: too low a fee).
He complains to you, "WHY IS THIS TAKING SO LONG?!!"
You capitulate after being manipulated; he walks away with the cash in his hand.
That same “customer” then shuts down his Bitcoin Node and the unconfirmed transaction is automatically voided by doing so. You will never receive the BTC. He scammed you.
...
Mins and Max
A common question regarding SELL settings in CAS is: “how to properly set the minimum and maximum mining fees?”
This is a complicated question. CAS uses (by default) an X rolling method of calculation. It looks at the last X transactions, averages them out, and forwards the SELL transaction using that average. This is where it gets complicated.
If the fee is rapidly moving higher, the transaction may get stuck in the mempool. This may happen on occasion when using the default settings. It’s inevitable. There are 2 approaches:
Wait it out. The transaction will eventually clear and your target wallet will be credited. Smaller transactions can usually wait in this situation.
Manually increase the fee. Sometimes this is appropriate, e.g. a large transaction that impairs your ability to do business. There are 2 types:
RBF (Replace By Fee) resubmits the same transaction to the network with a higher fee.
CPFP (Child Pays For Parent) creates a child transaction that forces miners to also include the parent in the block.
So, how do you avoid this issue to begin with?
You can set the SELL Minimum Mining Fee Per Byte and Maximum Mining Fee Per Byte settings. When you set these numbers, you ensure that your transactions are processed within a defined range.
Minimum Mining Fee Per Byte
Minimum Mining Fee Per Byte ensures that the transaction is never submitted to the network with a trivial fee. The vast majority of “stuck” transactions involve fees of 1 sat/byte (or even less).
Set too high: your profitability suffers.
Set to zero (default) or too low, and the “stuck” transaction will eventually happen.
Here’s a current graph of the mempool fees from https://jochen-hoenicke.de/queue/#BTC,30d,weight:
...
The bottom layer is “0-1” sat/byte. In the past week, most every transaction fell into this range, and many cleared during the week. Transactions over 10 sat/byte cleared within an hour (or less).
Between Sep 18 - Sep 24, the only transactions that cleared had fees over 17 sat/byte.
In this case, a minimum fee of 10 sat/byte may have been vulnerable to being “stuck”.
This vulnerability was only a problem if the CAS algorithm erroneously calculated a fee based on the last X transactions' fees not exceeding 17 sat/byte. In other words, the conditions would have had to be “perfect” for the CAS algorithm to fail to post a proper fee. This rarely happens, but it isn’t impossible.
Here’s a current graph of the mempool fees from the past year:
...
In this chart, a minimum fee of 140 sat/byte would have been vulnerable at some point.
And 6 years:
...
In this chart, a minimum fee of 300 sat/byte would have been vulnerable at some point.
The takeaway is: there is no “perfect” minimum. You will always be vulnerable as long as you are profitable. Paying 300x the mining fees to assure you avoid any delay is unreasonable (and expensive).
The best advice? Set it to 2 sat/byte (or slightly higher). This will mitigate the problem for most transactions, while retaining some level of profitability.
Maximum Mining Fee Per Byte
In the chart above, you can see that a mining fee of 400+ sat/byte was offered for some transactions.
At that time period, Bitcoin was around $4k, so a single fee could have been over $10. In some cases, the fees exceeded $20 per transaction - and Operators endured losses. You’re not in business to lose money.
Setting the Maximum Mining Fee mitigates your exposure to such problems. During a period of extreme fees, ALL your SELL transactions will get “stuck” - but you won’t suffer accidental loss.
Set the Maximum Mining Fee to a comfortable number that ensures your transaction consistently clear yet remain profitable.