Item price consists of a part with VAT and a part without

Item price consists of a part with VAT and a part without

For a beer site I have this small problem.
The beer is sold in boxes containing 6, 12 or more bottles
For these boxes (including the bottles) a guarantee must be paid, guarantee that is refundable after bringing back the box.
For this guarantee no VAT must be charged.
So total prce for a box of beer is price + %VAT + guarantee
If I put on the site price and guarantee, then the VAT is automatically added together with the standard price which is no good.

Any suggestions?

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