Nelly:
If what you say is true about the library then I cant use multiple shields if I'm using this specific shield? Is that correct?
The shield looks like it is intended for an Uno which has only one HardwareSerial port on pins 0 and 1. I don't know whether the shield will work on a Mega. The Mega has 3 spare HardwareSerial ports. I have no idea whether the use of the library affects them.
What do you want to use multiple shields for? I would always be wary of using multiple shields in case they conflict with each other.
...R