I would not consider an extra Re.1 as a "donation". It is very much part of the cash gift which we lovingly give. As mentioned by Varghese, it is 'shagun', that is, it is something auspicious for something good so that extra Re.1 is akin to a small extra wish. You may have seen on a birthday cake, too, people sometimes put an extra candle, for one additional happy year which is forthcoming, a way of wishing that the person celebrate another birthday.
In my home, we have been following the tradition started by our parents to put shagun of Rs.11/- at the deity's place for every occasion when somebody is going to undertake a new project, take up a new job, etc. Everyone present is asked to touch the money and then it is placed there. It is a gesture of seeking God's blessings and good wishes. We even put it when somebody is going to set out on a trip, seeking blessings for a safe trip, a happy stay at the destination, and a safe return. It makes me feel good inside each time I put shagun at God's feet. Shagun is really one of the most beautiful traditions to follow and I hope it never dies out.
When you make a commitment, you create hope. When you keep a commitment you create trust! ~ John C. Maxwell