It sounds like either, you clicked the button twice by mistake in which case LW.org will probably see the mistake & cancel one of them. Or you initially went to a different site where you paid the money & were then re-directed to the Official LW site to downld the Free version.

Hold on ... I just realised, you bought the LW CD which is that amount of money (have a closer look at that page!) The CD will be sent to you. Whereas the $18.88 fee is for the downld version. In any case I should imagine you will receive an email from customer support so check your mail (& your junk folder) to see if they have. Whichever email you registered with, use it to contact them HERE (click on link) to see if you can change the order if that's what you want.