PayOrder.cs

public async Task Handle(PayOrder cmd)
{
    // ❌ External side effect BEFORE we know our transaction will commit
    var receipt = await _paymentGateway.Charge(cmd.OrderId, cmd.Amount);

    using var tx = _db.BeginTransaction();

    _db.Inbox.Add(new InboxMessage(cmd.MessageId)); // UNIQUE(MessageId)

    var order = _db.Orders.Find(cmd.OrderId);
    order.MarkPaid(receipt.Id);

    await _db.SaveChangesAsync();
    await tx.CommitAsync();
}