mtgfanatic.com took mint condition collectible cards from me with a price quoted for payment that they later changed on top of which it took over a month to do Internet

About a month ago I sent in some very collectible cards to a company called mtgfanatic.com. These cards went straight from their new packaging to a plastic sleeve in a book full of these cards which I collect and do not play with!  Upon making the list of cards I was to send in to them I was quoted an amount of store credit I would be receiving for said cards. I have dealt with other retailers who sell and buy these same cards and have never had an order take over a week. These people take over a month to process my order, never returned even one of my emails and to top it all off they are now only giving me half of what they originally quoted. There is no way that they could rate these cards any lower than mint they are in as good of condition as if they were new. They are just totally ripping people off because they can, and something needs to be done!