If there's some unwritten rule about not using magic to help yourself (like many other magic books) then why does Mrs Weasley have a self-stirring cauldron? By Book6 they only have 2 kids to support.
And if Mr Weasley is so into muggle stuff why aren't they benefitting from Muggle conveniences that wouldn't contravene misuse. Why can't he say telephone if he's the biggest authority on Muggle artifacts?
Why didn't anyone just cut off the silly lace on Ron's 2nd hand dress robes in book 4?
The twins are also now successful shop keepers so they can help out the family. If they telephoned or used the internet, there would be no need for owls. There would be a huge rise in owl unemployment and the place would be riddled with owls hanging around and hooting menacingly.
I think the Weasleys like being the way they are and don't want to change. They've probably got vaults stuffed with galleons at Gringotts.