I find the voucher system fantastic, whatever the amount. For my birthdays I usually list the books I am interested in or ask for vouchers in an email to family because it is quick and simple to do.
I also send vouchers to other family members so that they can choose what they would like at their leisure.
When you know someone has a keen interest in a particular subject you can double up on a present because they already have it or someone else has bought it as well.
The voucher system is the quick, save way to give a gift.