If you are buying from a UK business that is VAT registered, then they should deduct the 20% UK VAT from the price when they ship it to Germany. You will then pay the 19% German VAT when it arrives, plus a fixed fee to the post or courier.
There shouldn't be any import duties, it will just be the VAT. I would suggest you talk to the seller first and make sure they understand that they should deduct it for you - most reputable companies will know what they should do.
If you are buying from a very small business that is not registered for VAT, or from a private individual, then there won't be any deduction from the price but you will still probably have to pay the German VAT.
Jon.