Merchant fees (credit card processing fee)

Hi, 

I would like to ask if merchant fees are under cogs or opex? My client is a small business owner providing HVac services. We are using HousecallPro for invoicing, recording of payments from customers (integrate with qbo). Most (majority) customers use credit cards when paying their invoice. In Housecall pro, the processing fee is under cogs as a default category and it's map in qbo as cogs. Please enlighten me. Thank you in advance.