Most companies ship in a plain cardboard box or padded mailer.
It'll look like any other parcel.
However, it will most likely have the name of the shipper on it, so unless your landlord reads who it is from, they won't know any different.
Some suppliers have names that can't easily be identified as a gun shop. Like . . . Wolverine Supplies, Questar, S&J Hardware, that could be anything.
If you're paranoid, I'd probably just avoid gun stores with Gun Store names, until things are moving smoothly and your landlord is used to you getting parcels all the time.
As far as if the suppliers will cater to you, that will be up to them. You could ask, some might be willing to print out a custom shipping label, some may not.
Then there is always the EE. It's a great place to get many things, and naturally the label will read as coming from another individual, could be anything.
Meanwhile, be warned, asking about discreet firearms cases may start a heated debate about legality.
It's illegal to "conceal" a firearm, so some will debate a discreet case might count as such in the eyes of the law and get you in trouble.
For small things like handguns, you can easily get a small case for them that you can toss in a backpack that doubles as your range bag and no one will think twice about it.
For long guns, if you're into any sports that the landlord is aware of, there are some soft cases out there that kind of look like duffel bags.
If it's a bag that is not designed for firearms, but is generic, as in not specifically designed for anything, and can enclose/lock the firearms adequately within the intent of the law, I say you're ok.
But if it's a case specifically designed for something, like a guitar, then you might raise and eyebrow if the wrong person found out.
For instance, getting pulled over, or a spot check, coming from or going to the range, and they ask you where you're coming from/going, you say the range, and they see a guitar case, and they jsut happen to be having a bad day . . . kind of scinerio.
Your best bet would just be to explore the various site sponsors, and suppliers on what they have available for gun cases and find one that doesn't look obviously like a gun case that maybe you can come up with a creative back story about if your landlord asks. Perhaps you decided to take up hockey or tennis ?
One thing to think about, is if it's ok or not to withhold this information from your landlord. Ideally it's none of their business, but if they own the property, well, I have no idea about that one, jsut something you might want to look into.