Why not for ease of counting just round it up to a tenner, Im sure we are not fooled into thinking along the lines of supermarkets with the 99 gimmick and a nice round figure would make it easier all round..
Also remember that even though we are paying £10.00 the amount the forum receives will be slightly less than that, In fact it might surprise you to learn you will get £9.40 give or take a few pennies, l just checked and for all my dealings with paypal that figure surprised me, However its fast,easy etc etc.
Im sure most of us would be happy to pay in full upfront and might be a bigger help...
The buyers can of course add a message on their payment invoice.