If you’re going to use the chain whip you just made, you’ll also need a way to get that cassette off. A lockring remover is a must-have; unless you use one of the shady removal methods I’ve seen on the internet—which I wouldn’t necessarily recommend unless you don’t care much about the condition of the wheel after you use them. I recommend Park Tool’s iterations here that cost about $6, but you’ll need to buy the specific one for your brand/type of cassette. (By the way, I’m assuming you already have an adjustable wrench laying around somewhere to use with this tool. If you don’t, well, you’ll need one of those too.)

The fabric attachment system is low-tech, but that seems to be helpful, as it can adapt to seat rails of different widths. One fancier option you might see on more expensive seat bags is a quick-release mount that you install under your saddle. But those have a fixed width, and therefore can fit under seats with only those exact specs. Brooks saddles, for example, are too wide for these mounts.
Mountain bikers are in different world of repair entirely, one that borders on the comedic absurd. It includes large pumps designed to fill up big, fat tires that squish over things, and a medley of assorted slimes meant to be injected into tubes or tires. In that world, the number of days it takes you to fix your tire and return from the wilderness is a badge of courage—bonus points if you’re bleeding—and we’re guessing that’s not what you’re going for next time you set out for groceries.
Yes, bicycle shops generally provide repair services. A typical list of offered services includes regular service/adjustment (including safety check; and  brake, drivetrain, and tire inspection); premium, and annual service, which is a more extensive tune-up. More specific repairs include drivetrain tune-up, brake system tune-up, flat tire fixes, brake repairs, cleaning and touching up the bike frame.

I brought my bike into Kyle's shop to fix the noise in my rear wheel. The mechanic suggested that I need to overhauled the rear hub for $30. The next day the noise came back even worst. I brought it back to the shop and they said it was the freewheel driver not the hub that needed to be work on and it would cost me extra $30 to overhauled it again plus the part to fix it. At this point I walked out of the shop and took my bike to David's world. The mechanic at David's world took a quick look at the rear hub and indicated that the axle was put back too tight when Kyle's mechanics were working on it and further identify the issue. I ended up paying David's world the $30 plus part to fix my bike, but the major different is that David's World has 30 Days services warranty claim. If you have any issue with your bike within the 30 days the service was performed you can bring it back and they will fix it no question ask.
Whether you ride a single, 7, or 10 speed bike or anything in between, you will need a slightly different chain. The major difference is that they come in varying widths to accomodate the different spaces between the gears, but quality of construction is also a factor in choosing a chain. Inspecing the chain should be a part of your regular cycle maintenance procedure- It’s important to keep a chain well lubricated, but oil will also attract dirt and so different oils for specific riding conditions are available. Remember to clean your chain regularly to get the longest life out of it and the gears.