It is good to see that the taxi driver called you to reclaim your lost item, it is actually the law that they do so. Anyway, it seems that you are looking for a cheap option, unfortunately, other than going back yourself all options are expensive.
- Ask the driver to come to Galle and pay on delivery.
- Ask on social media if someone is near and coming your way.
- Use a courier service like TNT or Pronto, they pick up the lost item from an address and can deliver it across Sri Lanka within a day or two. Alternately they can ship it internationally to your address. You can book the service by calling or from their website.