They ll ruin you every time. That might be good advice, but only up to a point. That point is reached when you go to contract. In a contract, you make commitments, which are the same as promises. You vouch for planned delivery dates, not random drop-off times.
Nothing is more important than customer service.
No customer service, and pretty soon, no business. In customer service, nothing counts like honoring commitments and meeting deadlines. That s why before you commit to anything, you search your abilities as an individual and as a company to deliver on that commitment. In sales, you must check with manufacturing, inventory, shipping, finance and so on. You must line up all your ducks before you put yourself and your company on the line.