Based on the chat, I think you are worrying about nothing.
Disclaimer: I am not a lawyer, nor an accountant, but just some random Internet user.
I suppose the main concern here is taxes. As long as you give the tax department the right numbers, you aren't evading taxes. If you mix up personal and business expenses, for example, it might be harder to prove they are the right numbers, so you might be making more work for yourself later if the tax department doesn't believe you, but they can't get you in trouble for tax evasion if you're paying the right taxes.
Successful businesspeople do not tiptoe around everywhere constantly worrying about whether they are doing something illegal. They use common sense; they ask their lawyers or accountants if they are not sure; if they do get caught in a mistake they suck it up and pay the penalty. If it's a legitimate mistake the penalty is likely to be quite small (but they still have to fix the problem and possibly also pay a fine). It is very unlikely that you go to jail for an actual mistake that doesn't hurt anyone.
It is quite normal for representatives of a business to buy things for the business on their personal accounts and then get re-paid from the business's money. It is also normal (but perhaps less common) for personal items to be bought on business accounts if there is a reason and the merchant is okay with it. But for example, if there is a different tax on personal and business purchases, you have to make sure the right tax is paid.
"Piercing the corporate veil" refers to a situation where the company gets sued, and the court decides that the company is not a real company and you should also be sued. This can happen if you are using the company to try and cheat a lawsuit - for example, you sign a contract saying you will not compete with another company, but then you start a company and say "I'm not competing - the company is competing." The court can say that actually, the company is you, so you are breaking the contract.
I am not a lawyer and cannot tell you exactly when the corporate veil is likely to be pierced. I don't think even a lawyer could tell you for sure, since courts make decisions on a case-by-case basis. However, according to Wikipedia it is likely to be a combination of many factors that make the company illegitimate. Mixing personal and business money is one of these, but the decision would be based on the overall pattern of how the company operates. It would not be pierced simply because you made one personal transaction with a business card.
Here's an interesting case from New Zealand in 1960 where the veil was not pierced. The owner of a crop-dusting (aeroplane chemical spraying) company died in a plane crash, and the courts decided his wife was allowed to get compensation just like he worked for any other company - the fact that it was his company made no difference, because he was not trying to cheat anything. He ran the company like any company should be ran, and he worked for the company like he would work for any other company, so there was no reason to treat the company any differently just because the worker and the owner happened to be the same person.
Of course, it might not be exactly the same in America, but it should be similar.
By the way, this is not something that destroys the company, if you were thinking that. Rather than "pierce" it is more accurate to say the court chooses to look behind the veil, during a lawsuit. The veil is still there and maybe it even won't be pierced in the next lawsuit.