I, too, have an older JoAnn's model, pd $100 and love it. Stable, with no shelves, underneath, but I store batting and a lifetime of "round tuit or dreaming of getting done" quilt kits waiting to be done under neath, so I can't see paying that kind of price just to get that one little bitty shelf. Check out JoAnns - they also will return at a store, if it was purchased on-line. Our Joanns has this table in stock - just saw it.
As for an extension behind your sewing machine - DH (who is not a carpenter) bought me a 4' countertop at Lowes, screwed 4 legs on with casters and it works great. Total cost $60. Might check out Habitat "Restore" I love that place. If its not there today, it will be next week - jusst keep checking back.
Bottom line - Walmart is great, but this time, way overpriced!