Transfer in case of mixed vAT

Dear all,

I have to carry out various jobs for a customer that are subject to different vAT rates. I split everything neatly by item and can easily separate hours and materials between the different vAT rates.

What about the travel costs I charge? Should these be charged at the highest rate of 21% or can they also be charged at 6% VAT?
The largest item on the invoice will amount to 6% VAT, but I suspect that this does not play a role. Thank you in advance!