South African - Tourist Voucher

Question

Hi,
I have a question re the required tourist voucher. We are two South African travelling to Russia. We will be staying in Moscow from 6-9 August and then on the evening of the 9th (or early morning of the 10th)  we will take an overnight train to St.Petersburg. We will then we be staying in St. Petersburg from the 10th to the 14th and will take an overnight train back again to Moscow in the evening. I have booked accommodation for Moscow and St.Petersburg and both places are willing to help me with visa support. However how do I support the fact that I will be on a train for two nights with regard to the visa application? I was wondering is it perhaps better to just get a tourist voucher from the approved agency on your site for $30?