please note that while there are dedicated Letter journals, like the ones mentioned already (in addition there is also IEEE Geoscience and Remote Sensing Letters), many journals allow letters a a form of submission. Often there is a "research article" for regular, long papers, but then there is technical notes/reports or letters for shorter ones. See for example in MDPI's Open Access journal Remote Sensing (http://www.mdpi.com/journal/remotesensing/instructions)
Vishal, yes, indeed, it is pen access and they charge processing charges. The reference to "Remote Sensing" was only an illustration of a geoinformatics/ RS journal accepting letters as well. Also papers in journals from Elsevier or IEEE that you cite are often not free.