I received a text claiming that I came in first in a Walmart giveaway, based on a unique code printed on my Walmart receipt (which I do not have). Is this a scam?

For sure that's a scam. You can double check with customer service by visiting/calling/online

Yes obviously a scam. Any text you may get about a parcel delivery are definitely a scam also.

I think so. To be sure you may call Walmart customer service and ask them.

Most likely. Unless you gave your cell number to the clerk at time of purchase.