This is my version of script based on what you requested for originally, but there's a drawback, that is, it can only calculate the total cost of two items purchased. I think I've covered all of the trigger commands you depicted. Nevertheless, this script could be further improved.

Code:
alias bill {
  var %b = %x + %y
  return %b
}
on $*:TEXT:$(/[!](coffee|chips)/Si):#: {
  if ($regml(1) == coffee) { 
    msg # that will be 1.99, do you want !white or !black?
    set %coffee coffee | set %x 1.99
  }
  if ($regml(1) == chips) { 
    msg # That will be 0.86. Thank you.
    .timerchips 1 3 describe # Bot hands $nick a plate of chips.
    set %chips chips | set %y 0.86 
  }
}
on *:TEXT:*:#: {
  if ($1 == !bar) { msg # Welcome $nick $+ , welcome to the bar! On today's menu we have, !coke , !coffee , and !chips }
  if ($1 == !milk) { describe # Bot hands $nick a teaspoon of milk. }
  if ($1 == !sugar) { describe # Bot hands $nick a teaspoon of sugar. }
  if ($1 == !bill) && (coffee isin %coffee) { msg # $nick $+ , you bought %coffee which cost:  $+ %x | msg # $nick !cash or !card ? }
  if ($1 == !bill) && (chips isin %chips) { msg # $nick $+ , you bought %chips which cost:  $+ %y | msg # $nick !cash or !card ? }
  if (coffee isin %coffee) && (chips isin %chips) { 
    msg # $nick $+ , your total comes down to:  $+ $bill | Do you wish to pay by !cash or !card ?
  }
  if ($1 == !cash) { describe # Bot took $nick $+ 's cash for the purchase and gave $nick a receipt. }
  if ($1 == !card) { describe # Bot took $nick $+ 's card for the purchase and match $nick $+ 's ID and signature. }
  elseif ($1 == !bill) && (coffee !isin %coffee) && (chips !isin %chips) { 
    msg # $nick $+ , you haven't bought anything yet. 
  }
  if ($regex($1-,[!](black|white))) {
    if ($regml(1) == black) { 
      describe # Bot hands $nick a cup of black coffee.
      .timerwb1 1 3 msg #  Any !milk or !sugar? 
    } 
    if ($regml(1) == white) { 
      describe # Bot hands $nick a cup of white coffee.
      .timerwb1 1 3 msg #  Any !milk or !sugar? 
    }
  }
}