Weigh that with times that they pay in advance. It's a case by case bases and I'm sure they're not late with payments. If they wait as long as 90 days, I bet it's because that was part of the business agreement they made with the manufacturer.
Paying late in manufacturing is all over the place, 30 days is considered acceptable, 60 the absolute limit, but paying 90 days late is just something Apple can get away with because of it's size and "occasional ruthlessness".
It's also not unusual for companies to pay later than the agreed dates, but no-one has said Apple's doing that.