It's probably worth checking with Blackburn regarding their "lifetime" warranty. It's an 800 number so it would be worth trying: To make a claim, call our Customer Service at 1-800-456-2355 for a Return Authorization(RA) number. Some issues may be resolved upon the initial call. You might not need the original sales slip to get a fix.
It's funny that this $25-$30 light would fail so quickly when there are so many inexpensive next-generation 100 lumen COB tail lights that cost under $10. I consider them to be "throw away" when they eventually do die as all Lithium polymer battery lights will. I've had a pair of them for about a year and they don't lose much charge if left for even a few weeks. Only you can decide whether or not it is worth the shipping cost to get the light fixed. It might be as much as the cost of a better new light.