I use Authorize.net for my Shopify shop. I would prefer to use Shopify's own payment processing (which is via Stripe), but because I sell dietary supplements, I'm prohibited.
Authorize.net assigned me the Merchant Service Provider (MSP) of Cybersource. I didn't realize this was going to be the case - I just assumed that everything is done via Authorize.net.
After potential customers couldn't pay via their AMEX cards, I asked Cybersource what's up and they said I need to get a separate American Express merchant account. I though the whole point of utilizing Authorize.net is so that I don't need to contract directly with credit card processors...
I'm not sure whether I should stick with Authorize.net (I believe I already paid a setup fee). I'm limited because some won't do dietary supplements. Does anyone have experience with other MSP's I can choose? Authorize.net tells me that if I switch, it's going to be another 10 cent per transaction..