doesn't Discover charge retailers a higher percentage on purchases compared to Visa or MC?

a cursory google search yielded:
 
https://www.fool.com/the-ascent/research/average-credit-card-processing-fees-costs-america/
 
tldr, Discover and AE charge more, plus they use their own proprietary payment network, which further complicates transactions. 
 
paying with Paypal is an option. Maybe you have to link your Discover CC to Paypal and pay that way?

I can count on one hand the number of retailers in my town that take Discover.  Where do you think that 1% cash back comes from?
I guess EVGA could charge you a processing fee to offset the money you don't think they deserve.

Ever been to Costco? Did they take your Discover card?

Bottom line is that the bottom line has a lot to do with card acceptance, and Discover is expensive for merchants to use. Mega chains like Walmart can absorb it. Smaller merchants with tight margins can't. Some larger chains, like Costco, also won't take it (or AmEx) because of the higher fees and proprietary networks.

You could probably go buy a prepaid Visa and pay with that. I seem to recall that others have done so.

Either way, it pays to verify your payment method is accepted at a given merchant, before you get to the register and end up embarrased.

As a business owner, we hate when people use Credit cards to begin with.  All those points and cashback rewards are paid via fees.  Visa and MC are the lowest, about 1.7% plus or minus a couple tenths.  AMEX is absurd, 3% or more.  Discover is I think like 2.5%   It is VERY common for merchants to not accept certain payment methods.  AMEX is the most frequently not accepted card.  Discover is next.  We will discount a purchase for cash payment. (most of our transactions are $1000 or more so you are talking about $25 for a Discover card, $30 for AMEX.....or $17 for Visa and MC.  There is also sometimes a small "transaction" fee for each sale of like 39 cents.  As the other poster said, Costco takes only one card.  Don't have one, get one or shop elsewhere.
Paypal is the worst.  They charge 3% plus fees.  In a year my sales on ebay would have several thousand dollars in paypal fees.  Same with Square.  These were designed for people to small for a processing account in the early days, or small guys looking to avoid paying uncle sam.  Nowadays anyone can get a processing account and uncle sam micro analyzes transactions.  Bottom line, get something with a Visa logo or be willing to pay cash.