Melling is a well-known brand name in the automotive industry and I have used their stuff for years. But everybody makes mistakes and this time, you just happened to be on the receiving end of someone's error.

Box the lifters up and return them to AutoZone. Double check the applicable part number with the counter guy and inspect another box of lifters at the store before you leave. Take an old pushrod with you and your dial caliper to check 'em out before you leave the counter.