Leaving it untill 8 minutes before the deadline when the store shuts is cutting it a bit thin. The bloke who works there probably wants to go home at the end of his shift just the same as the rest of us & just because you said that you would be there doesn't mean diddley squat.
Saying it is one thing, doing it is another & a very substantial percentage of callers that say that they will call in, ........... don't.

If your dealings at the store took, say, 20 minutes, would it be OK for him to ask you to leave at the 8 minute mark [12 noon] since he would be 'off the clock' at that point, or is he expected to work on for free, then go through the packing-up & closing-up shop procedure.

If you want to do this stuff on a Saturday morning, then starting at 7:30am is a much better idea as it gives you a much better time buffer.

Expecting general parts outlets to stock long U bolts for use with lowering blocks is really a bit of an ask. They are not really a big turnover item any more in these days of front drive & independant rear suspension cars, & stocking slow or non moving parts is simply not smart from a business perspective.
Dismantling a car before you have all of the required [specialised] parts to complete the job late on a Saturday morning is not too smart either, as you have now discovered.

Think twice, act once.