I'm a recently formed, small startup business as well. I was originally going to do the old fashioned check and cash method, but after thinking about it I realized that having the option to accept credit/debit cards would be a huge advantage. Yes, any payment processor is going to charge a fee for each transaction, but most aren't that expensive. A small 1-3% of each transaction done with a card is worth it to me, seeing as how many people don't even use checks anymore and others don't want to carry a large amount of cash on them, as well as some people who want the security of having the backing of their bank as a security of their purchase. From the perspective of a customer, the only time I won't use a card to make a firearm purchase is when I need to purchase first and then tell my wife afterwards, once I can show her the justification for it
Many banks also offer payment processing abilities if you have a business account with them, although they tend to charge higher fees and I wouldn't personally consider going this route unless I'm making a ton of transaction with a high revenue. Some POS companies also will charge a flat monthly fee instead of, or in addition to a per transaction fee. I ended up signing up through gunpos.com and getting a free POS device. Another advantage of this service for me was the fact that the clover pos system can integrate seamlessly with the FastBound service. So I can conduct a sale, background check, and 4473 all in one place. I'm still waiting for my FFL to come in mail so I haven't been able to use it yet, but I've been experimenting with it in the mean time and I already am a fan of it.