The functional ones matter first and they are where trust is either built or lost. A confirmation sent immediately, containing what was ordered, what it cost, when it will arrive, and how to reach you. A shipping notice with tracking that works. Anything needed to use what they bought. The absence of these produces support contacts, which are expensive, and anxiety, which is worse because it precedes the first review.
The interval after purchase and before delivery is where doubt lives, and it is the period most businesses leave silent. Somebody who has paid and heard nothing for three days begins wondering whether it worked, and that wondering is what produces both support messages and chargebacks. A message in that gap saying what is happening costs nothing and prevents both.
After delivery, the message that produces the most value is the one asking whether it arrived well and whether they need anything. It surfaces problems while they are still fixable, which is the difference between a complaint you resolved and a review you cannot. It is also the natural moment to ask for a review, because the value is present and visible.
Instructions and useful material outperform promotion at this point. Somebody who has just bought is receptive to help using the thing, and not receptive to being sold something else. A message helping them get more from what they purchased builds the relationship that makes the second purchase possible, and a message pitching an upgrade three days after delivery reads as though the sale was the only part that mattered.
Sequence these deliberately rather than sending everything at once. Confirmation immediately, dispatch when it happens, arrival check a few days after delivery, and a review request once they have had time to form an opinion. Four messages, each expected, each useful, and none of them asking for anything until the last.
Test the whole sequence by buying from yourself, including on a phone, at least once after any change. Confirmation messages break silently when something in the checkout or the email settings changes, and the failure is invisible because the people affected do not tell you, they simply worry and then contact support about something else. A single test purchase a quarter catches it.