Now that everyone has very cleverly made the points, in the comments, that I made myself in the actual conversation, I'm going to reiterate them all here.
The first thing I suggested in my chat with the Milk Guy was that, if there is indeed a God, He would not be subject to time, and so therefore His helping out with a shopping trip wouldn't exactly waste any for Him. But I also got the sense that asking for help with something as seemingly shallow as a shopping trip was perceived as just a little audacious, beyond the issue of time.
I conceded that, say, praying about my cancer diagnosis is probably more important that praying about the results of my shopping trip.
("You think?" said the Milk Guy.)
But it seems to me that if there's a God, it would either make sense for Him not to care about anything that concerned us, or to care for everything that concerns us. He's God, not us, so who decides where the line is regarding Important Enough for God? He has every right not to care about my shopping trip--but given my relative importance in the universe, I would expect that if He didn't care about that, my ex-tumour wouldn't be that big of a deal to Him either. In which case I should just not bother to pray at all.
Since, however, I believe that God does care (based on my understanding of the Bible), then who's to say He doesn't care about my shopping trip as much as my healing? I remember when I was a teenager and first became aware of I Corinthians 10.31: "So whatever you eat or drink, or whatever you do, do it all for the glory of God." I remember feeling completely flummoxed by the concept; how could I glorify God by eating a sandwich? Of course I know now that there are a lot of ways to live that verse out in a lot of contexts, and I feel it applies here, too.
Sure, I can pray about everything so much that I paralyse myself and become incapable of making the simplest of decisions. But I think in this case, asking God to give us a successful shopping trip opened us up to being able to glorify Him in that shopping trip; as my mother pointed out in the comments, though God is not bound by time, we are. We had limited time and, in spite of a financial gift, limited resources, and to me it makes a whole lot of sense to talk to God about the endeavour first, so as not to waste what He had provided us with.
And that's another thing. Jesus told us not to worry about anything, particularly food and clothes, because our Heavenly Father knows we need them and He will provide them for us. But He did tell us to ask that same Father for good gifts. The Heavenly Father had already provided my mother and me with the means to supply these specific needs, so it seems a logical next step to ask Him to help us find the things He had, essentially, already given us.
("Fascinating," said the Milk Guy.)
I do recognise in myself a tendency to pray primarily for myself, and maybe one or two others who are particularly close to me. I don't think this is the right way to pray. I agree that there are much more selfless things to pray for than even my battle with cancer. But I don't think that means I should pray for the minutia of my own life less. I think I should just pray for everybody else more. If God clothes the flowers, and if He knows when a sparrow falls to the ground, and if it's true He wants a relationship with me, then it may be audacious to ask Him for a good haircut or something. But I believe He loves that kind of audacity, and in gratitude, I'm just going to keep telling Him about and asking Him stuff.