A callout is a minimum charge on top of the first hour's labour to get to the property, essentially to pay for travel and travel time and, realistically, the time spent doing paperwork. Generally, I think we all tend to include this inside the minimum charge for up to an hour's work.
Time spent getting some extra waste pipe because there isn't enough in the van should be covered by whatever mark-up the plumber puts on materials. It's a standard part and if there isn't enough in the van then we've misjudged our stockkeeping.
But what if the customer needs a new 3-port valve head? They are manufacturer specific. Would it be reasonable to call them a standard part? A ceramic insert for a tap. They aren't all the same. The replacement heads for TRVs are all different and rarely compatible. This kind of thing is where I might be charging an hourly rate for time spent at the merchant (though I'd make it quick) because I cannot reasonably carry all this stuff in the van. Pimlico Plumbing does much the same. That said, the charge for the time spent is not as much as another minimum charge.
What/how much is Mary O being charged though? I might have a different price structure that sounds fairer, but might still be more expensive. So the bottom line is relevant.