Here is some traveller information about crossing the border at Chiang Khong – Huay Xai:
We went to the Thai border post in the north of Chiang Khong to get an exit stamp. Then we walked down to the river, paid 40 THB per person for the short boat trip across the Mekong to Huay Xai, Laos. There we filled out some forms and applied for a visa on arrival for 30 days. Additionally to the visa fee (for Austrians 35 USD, see picture) we had to pay one USD overtime fee, which applies on weekends and after 4 pm. It is advisable to pay in USD because the fee in THB is much more expensive. Furthermore you need a passport photograph, but there’s a photo booth at the Thai side.