The mark on the card is the official signature for the character. Weiss Schwarz is releasing character signed cards like that one. Niko's name in her signature is also spelled as Nico.
SIF's release of cards also spell Eri as Eli and Niko as Nico.
I think 絵里 is romanized as Eri because Japanese don't use "L" in speaking, and にこ as Niko since "C" and "K" sound the same. When I type Japanese characters, and I used "C" instead of "K", it produces the same character.
Hmmmmmmm, the thing is, a lot of 3rd part listened stuff go between these two spellings, so I'd say there is no need for right now, but, if you can find references from the anime or official Lantis merc then maybe we should change it. That is my opinion on this topic, but I will give the final say to Eth.
I will forever prefer the ERI spelling and even the earilest images of her name found in the Mogyutto love de Sekkin Chuu PV, which premiered in February 2012, clearly shows from the very beginning her name was spelled as Eri. The 'L' spelling was subsitute later on for reasons entirely unknown and will forever create confusion over the spelling of her name, but even as of February 2013 the title of Episode 7 is spelled "Erichika" and not 'Elichika'...