It would also depend on the process that you will later use to isolate the RNA from the tissues but my choice for storage of tissues, prior to extraction or after homogenisation, would be TRIzol (or analogous Guanidinium thiocyanate-phenol-chloroform TRI reagent) to ensure the stability of the RNA.
See "Cancer Epidemiol Biomarkers Prev. 2010 Oct;19(10):2445-52
Effect of long-term storage in TRIzol on microarray-based gene expression profiling".
RNAlater is fine, but as your samples are already frozen there might be a problem. the qiagen manual says:
"Only fresh, unfrozen tissues can be stabilized using RNAlater RNA Stabilization
Reagent. Previously frozen tissues thaw too slowly in the reagent, preventing the
reagent from diffusing into the tissues quickly enough to prevent RNA degradation."
you could use trizol (add the trizol to the samples on dry ice without letting them thaw, then grind them thoroughly). but as claudio mentioned, you might have problems shipping that.
"RNAlater®-ICE transitions frozen tissue to a consistency that is compatible with
homogenization [...] Soaking frozen tissue samples overnight in RNAlater®-ICE at –20°C [...] changes the physical state of the tissue from brittle to pliable at –20°C, minimizing RNA degradation."