Just create your own function in standard module, and use it anywhere (in queries, in calculated controls, in vba code, etc.) you want.
P.e.
Code:
Public Function udfMyEtsyPrice(parValue As Double, parPerc As Integer, parCoef As Double, parRnd As Integer) As Double
' <Enter any code you need to calculate before calculating returned value>
udfMyEtsyPrice = "<enter your expression for returned value here>"
End Function
You call it as any other function, like
= udfMyEtsyPrice([Etsy Price], 11, 0.6, 0)