I think you might want to tell your doctor all the problems you have been having and see if he wants to order a nebulizer for you. A nebulizer treatment gets down much deeper in your lungs than an inhaler ever can. They now make portable ones that cost around $100. You can even use it in your car, it plugs into the cigarette lighter. Having this smaller one that I can use anywhere has given me a much more secure feeling. I'm no doctor, but it sounds like to me you need a nebulizer.
I use Xopenex in my nebulizer and they have now started making it as an inhaler. Oh my gosh, I've gotten out in the cold several times this week and started coughing and wheezing and would have normally have had to have gone to my car to do a nebulizer treatment, didn't have to do it. I was in Walgreen the other day and started coughing and wheezing and couldn't stop. A young girl who worked there came up to me and asked if she could get me a bottle of cold water, I told her that would help, that I had asthma. She told me she knew I did, because she also did. She brought me the bottle of water and then asked me if I had an inhaler with me and I used my new Xopenex inhaler and I was calmed down in no time. She was my angel looking out for me that day. I don't know if you have noticed, but when my breathing is not working right, neither is my brain.
As for Wal-Mart, I think you are going to have to have your doctor write them a letter stating what your limitations are and that you may have some absences due to the severity of your asthma, which is out of your control. Whatever you do, don't let them upset you to the point where you just quit. Get a doctor's excuse to take some time off before it gets to that point. I'm hoping not, but it could get to the point that you end up on disability at Wal-Mart's expense, so keep your cool. I know that's hard to do when you feel like crap and I also know employers are not very nice to you when you miss work. There's also something called FMLA, Family Medical Leave Act which allows you to take up to six months off work, without pay. You have to apply for it and your doctor has to fill out forms documenting your illness. You don't have to ask for the entire six months off. You can ask for two weeks, a month or whatever you think you need. I'm just trying to let you know what your options are.
The reason I asked you about lifting was because I didn't figure you were able to lift very much. If your asthma is very severe, you just don't have the lung capacity to do so, believe me I know.
I'm here if you need me.