If it's over about £20 and customs spot it there will be a charge, now I don't object to paying the VAT it's the Royal Mail 'fee' they add on my behalf because they paid the VAT for me - despite the fact I was never given the option to pay up....anyway - Q1 yes, if they spot it you will have to pay.

As for 2 - You won't be entitled to anything because the postage was a flat fee - unless they have a really good customer service and they ship your second bottle for free.

If you play up the fact that you weren't contacted - you may get something