It sells you everything at $39 and under that price. However standing orders are a queue system, and if anyone else has a SO for food at $39 or more as well, then you and those people will take turns to buy any food that comes along at $39 or less.

Imagine you have a queue of standing orders like this:

1st: (A) $39
2nd: (B) $38
3rd: (C) $40
4th: (D) $39

If some food comes along at $40. The $40 food will skip past (A) and (B), then try to satisfy as much of (C)'s order as possible. If C's order is not filled completely, C gets moved to the end of the queue:

1st: (A) $39
2nd: (B) $38
3rd: (D) $39
4th: (C) $40

Then if $39 food comes along, it tries to sell to (A) first since it is at the start of the queue, then (D), then (C) in that order. Any SO that purchases at least 1 unit of goods gets moved to the end of the queue. SO lets say (A)'s order gets completely filled, A's order disappears, then D's order is partially filled, it gets moved to the end (C doesn't get moved to the end because C didn't get to buy any unit, the amount of food sold wasn't enough to satisfy D's order completely):

1st: (B) $38
2nd: (C) $40
3rd: (D) $39

Now B's order is at the start of the queue. Basically, the cheaper SOs eventually filters to the start of the queue, but if they end up buying a low amount of goods, they still get thrown back to the end of the queue.

If a SO in the queue doesn't buy anything for 72-96 hours (random duration), it also gets moved to the end of the queue. This prevents gaming the system with setting early $3 SOs have it be at the start of the queue all reset long, and have other people sell to this SO at $3.

For this reason, it is better to have 3 SOs out if you want to buy _everything_, since you have 3 spots in the SO queue.