Unless you come by a very circuitous route, getting to and from Champasak will involve a crossing of the Mekong as the town sits on the West bank and the transport options are all centred on the East. I arrived in Champasak on a public jeep bus affair, where I counted 31 other souls, assorted poultry, a boxed TV set and apparently half of the foodstuffs in the Province. It was uncomfortable to say the least but is only a couple of hours from Pakse. Getting away either North or South will involve a similar river crossing connecting to either a local bus or minibus. Any guesthouse in town can arrange onward transport to just about anywhere including Bangkok and various destinations in Vietnam, should you require to go that far, and they will arrange a pickup to take you to the boat.
Be aware that if you get a smaller local (i.e. passenger only) boat from Champasak, you may not be landed at the pier and may well find yourself, as I did, tramping over somebody's vegetable patch to get to the road.
The photo is of the larger boat crossing the Mekong to Champasak.
At Downhung Market there are mini truck or Songtaew to Champasak city, start 2 times a day, 7.00 am and 12.00 am.
If you get Champasak and wanna return to Pakse in the same day, there are no bus or Songtaew back to Pakse, you should stay there.
One vehicle which I always choose to travel. I hired the bicycle from one guesthouse in Champasak. It costs 50 THB per day. Then, I rode it to Vat Phou and surrounded area. It was a good trip with bicycle.
From Pakse to Champasak,it takes about 33 Kms. at the end of trip you need to cross the river by ferry. I don't know how much per car,cuase I used the public bus which included the ferry fee already. To cross the river by ferry,it takes about 30 minutes for both queing and foating time. When on board,you 'll see local sellers come with thier food products. You can buy Pho noodle,steamed corn and pea,... Apart from shopping, I think it was a good time to see a beautiful scenery of Mae Khong River.