There's no need to shop online for a shotgun, or many handguns online. The FFL you ship it to about always charges $25 for the transfer, and you will be paying maybe $15-$30 for the shipping from the website as well. IMO, buying guns online is for guns that are hard to find, not trying to get a better deal.
Dick's Sporting Goods by me is selling Mossberg Maverick 88's with 18" barrels for just $199.99 right now, so maybe $210-215 after taxes. 12 Gauge ammo is not too pricey, birdshot for pests and practice runs about $5.50/25, compared to $10-13/50 for 9mm, so about the same. A handgun is semi automatic, so you'd probably be shooting more with it as well, so I'd dare say that ammo cost is a draw or in the shotgun's favor. Buckshot is $5/5 if you are lazy, and can be had online (try Sportsman's Guide, I'd suggest Fiochi Reduced Recoil 00BK) For like $6/10.
You chose a good time to buy a gun- Save up your money until Black Friday, get up at 5 AM, and head to your local big sporting goods store and get a Norinco/H&R/maverick pump for $150-$180.
Other, maybe better option- use google maps and search out all the Gun Shops within 50 miles, and then spend a day running around checking things out. I'd say 99% chance you could get an H&R single shot for under $100, or a pump shotgun for under $200. I you have the money saved, you can compare the feel in your hand of various handguns in the $300-$450 range- lots of good choices. Try Ruger P95, S&W Sigma, S&W SD9VE or SD9, CZs, Sig SP2022, Ruger SR9, ever Rock Island 1911s. You could look for used S&Ws, Glocks, and such for that price too. Pretty much any of those will last you a really long time, and many of them are light enough for IWB carry if you're interested.