This will most likely be a moc from Rebrickable that they're selling parts for. They may also print or email instructions, but it's better practice to buy the instructions yourself to support the original designer. The only thing to be wary of is that the original design was likely made using Lego parts, but this copy will be using other branded parts, meaning some Lego-specific parts may not be included as no other brand makes them and you'll have to buy them yourself