You haven't told us how long you worked at Nordstrom for. This is important - depending on how long the job was for will impact it's usefulness on your CV. If it was for more than 3 months, I'd keep it in (regardless of the reason for termination). If for 1-3 months think about it, and for less than 1 month I wouldn't really bother.
Less than one month and I don't think it is very useful to put a position - it just seems odd to have a job for that short of a time.
Being laid off for "business fluctuations" is, as has been pointed out, different from being fired for a disability. The fact is the latter is actually illegal in the US, which is where you appear to be from.
You are worried about "being fired". Rest assured, being laid off due to business fluctuations is vastly different, and as this is the reason you were given you should suffer no ill-consequences.
Anyway, leave the position in (or not, depending on how long for), and list the key skills that are relevant for this new position you're applying for. If that involves manual work, then note you lifted shoe boxes, for example. If it involves working with others, note you enjoyed helping customers and working with your team mates to address customer requests. Perhaps note your enthusiasm for hard work and can-do nature let you sell ~1K shoes/day.
Do not mention in your CV that you were laid off - just list the dates and the relevant skills. That is all that is expected from anybody.
What happened at Nordstrom?
In any event, if asked why your Nordstrom job ended, you can say because it was temporary and based on seasonal requirements - which is true. The most UPS will do to check is check with Nordstrom if you worked the dates you said you did. Nordstrom certainly won't say "we fired this person because of a disability" because, again, that is illegal. They might say "the role ended due to seasonal fluctuations", because that is what they told you, and, also, a legal way to end a role. That reason, happily, does not disqualify you for any future roles anywhere.
Please don't mention to them your fears that you were fired because of your "working memory" issues. It is neither true, nor a useful thing to reveal. Rest assured Nordstrom, again, won't reveal this should they be contacted.
Finally, if you are still in college, if you haven't already do check with your student services. They should offer the medical care to work help work out this "working memory" issue you mention. This might be anything from undiagnosed ADHD or dyslexia, or a raft of other things. I'm not aware of any particularly untreatable ailments these days, I'm sure there is something to help you. You owe it to yourself to make this your priority.
Failing that, there should also be council and government services to help you with this - I don't think it would be fun to be going through life feeling that you are in some way impaired, and offer you encouragement and the best of luck in finding a resolution.