I'm going to refuse to go on an expensive trip my grandmother's planned
How can you tell your little grannies no?
I think it's wrong to spend lots of money on things you don't need so much
I think that they invited you because it is important to them that you go. I think that you should just agree to it so that you do not end up hurting their feelings.
They will get over it when I tell them no I don't want to go and my granny can save her money.
I think it is safe to assume your grandma really wanted you to go if she invited you. Refusing the invitation will probably hurt her feelings.