RadioShack’s $50 iPhone discount promotion was, according to the company, wildly successful. How successful you ask? The Shack’s PR team hit us up to boast that a “record setting number of phones” were sold earlier in the month. And what better way to stave off the holiday sales hangover than to keep the discounts coming, no? “Starting Wednesday, December 29, RadioShack will offer $50 off every iPhone 4 16G and iPhone 3G S for new and eligible upgrade customers while supplies last,” reads an email sent to BGR. If you happen to be upgrading from an iPhone 3G or 3GS, The Shack is offering additional savings to those willing to trade in your old iDevice. “Qualifying iPhone 3G and 3G S handsets will have a guaranteed trade-in credit of $75 and $125, respectively.” To qualify your device has to “power on, cannot be PIN locked and must be in fully functional, working condition without any broken components.” iPhone 3GS upgraders would be able to acquire a 16GB iPhone 4 for a measly $24 after the $50 instant discount and $125 trade-in credit. Not too shabby. The deal does not have a set end-date, but RadioShack does note that interested parties should call their local store to check product availability.