RadioShack dropped the price of iPhone 5S to $100 and is also letting customers trade-in old iPhone 4S for a new iPhone 5S with no upfront costs.

RadioShack is offering a new deal for customers looking to buy a brand new iPhone 5S. The promotional offer, starting Friday, lets customers purchase an iPhone 5S for just $100 or trade in their existing iPhone 4S for a 5S with a waiver of the upfront cost.

 The struggling electronics retailer's new offer has a catch, of course. The trade-in offer for a new iPhone 5S 16GB version is for new customers willing to sign a two-year wireless contract with AT&T, Verizon or Sprint. The promotional offer extends to those who are already tied to a contract with carriers, but should be eligible for an upgrade by contract renewal.

Normally, an iPhone 5S costs $199 with a new or renewed two-year contract. RadioShack's offer drops the price down to half with contract. The unsubsidized value of the iPhone 5S remains the same, $549 without a contract. The promotion was first reported by 9to5 Mac before it was publicly announced Thursday.

For an older iPhone to be eligible for trade-in, RadioShack will inspect the device's working condition. If the device meets the eligibility criteria, the retailer will offer a credit of $100 that will be applied toward the iPhone 5S. This way, customers will not be asked to pay anything upfront, but as per the contract rules, the remaining cost of the phone will be recovered with customer's willingness to commit to a carrier for the next two years.

Currently, there are several financing offers that come directly from carriers such as AT&T and Verizon. Simple financing plans break down the cost of an iPhone into monthly payments so customers are exempted from paying huge upfront costs. But a $100 discount is a catch as it can get an iPhone 5S over 5C.

There is no word on how long the promotion will run, but it is certainly not going to be a permanent price change. Customers can either take advantage of the deal online or from any retail outlet.