It's a strange question because you're assuming what your mother will think. Anyway, it more or less works like this: "My mum will love that postcard." = I think so. "My mum is going to love that postcard." = I'm pretty sure of this, when she gets the postcard.

You use going to, when you're describing a fact in the future or when you see the logical consequence of doing something specifical. So if you are sending your mum a postcard, it's logical that she'd love that postcard. You know that already. You use "will" when you think of something, but you can't be sure. Like a prognosis of something. Or you use it if you decide something spontaneously.