Sod making them wait, don't pay at all. If they had said that FIRST time you sent it in, then they would have a case but they didn't so they don't. They cannot hold you responsible for the fact that they didn't repair it properly first time. This would apply if it was a fault first occurring now, however since you had the computer 'repaired' and it was a botch job, you are now sending the computer back to correct THEIR mistake. It's nothing to do with you, the responsibility lies with them because the fault is theirs.
At least, that's the way I would argue it. I certainly wouldn't pay. In fact it sounds distinctly like they are trying to avoid you sending it back to them.
