You can cut it. The leftover cut-away orphaned piece will work with another power supply, but it's not an easy thing to solder that joint properly. Wouldn't it be cheaper to buy a shorter lenght?
Or double-triple-quadruple wrap all of the wire's length around your backpack?
Lastly, make sure to order from a source which ships from within the US. EL is supercheap when it is shipped from China, but at this point, it will not arrive on time.