For many "pay by GC" sites, there is an intermediary site that monetizes the GC for customer / service and takes a fee for the honor to do so.  
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On TER, go to the "Buy VIP" dropdown. Select "Trade Any Gift Card for VIP Membership". TER uses paygarden, www.paygarden.com to read more.  
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The fees for each card brand differs. Paygarden doesn't want to get stuck holding cards they can't easily resell so you will get less for something highly specialized than a generic or popular card. Target is pretty safe. If you buy a $50 Omaha Steaks GC anonymously for cash at Walmart, that might be redeemable for $40-$45 worth of fees at TER or SA or whoever accepts payments via paygarden.  (I don't know the actual exchange rates. It might be a lot more than than my made up example.)
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Visa, MC, Amex cards are prepaid CCs, not GCs. As long as SA is not overseas (US regs make it really hard to use prepaid CCs for cash transfers or payments internationally), you should be able to just pay as if it's a normal CC.  
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I hope that better informed people will provide answers that apply to SA.