How many purchase cancellation can a seller make?

[fashion*essence]

I have a lot of NPB lately and for sure those cases I open a NPD to get my FVF back. Other cases where the buyers politely ask to cancel the purchase and apologise, I cancel the transaction to get the FVF back.

My question is, will there be limit for cancellation?

Because if there isn't a limit, people would just try to do a lot of cancellation to get their FVF back, even the item is actually sold.

I am worried that if I do a lot of cancellation out of my kindness, then I'll be disadvantaged myself sometime later in the future.

On another hand, I can't say no to those who are so polite and apologetic for changing their mind. I won't be able to open a NPD on them. But if there is a limit, then I will not cancel those cases where the value is small.

Any information is much appreciated.