Thought I'd provide some advice on this topic as we couldn't find any recent information about this. We just travelled from la Paz to Santa Cruz in Bolivia and were preparing for a nightmare bus ride as busses in Bolivia don't seem to be too crash hot. The experience was the exact opposite. We ended up buying a ticket with a company that we were recommended by a bus employee in cusco. The company we went with was called trans cococabana. We paid 170 bolivianos and got on a semi cams bus that had 3 rows, which is great. It was a double decker bus of the same quality of buses in Argentina and Peru. Bus left 7 pm and got to Santa Cruz at around noon. No food was provided on the bus but stopped at a breakfast place I stead which served great local food. I. Short this was the best bus ride we had in Bolivia even comparing to some tourist busses.
Note that at the bus terminal there are 2 companies trans cococabana and trans cococabana emp #1 we took trans cococabana sa but think the emp #1 is great too.
Please note that there is also a ,tourist, bus you can book through kanoo tours for twice the price 50 us. Can't comment on this service but it might be an option.
Other option is tam Bolivia whi fly together with boa. Note that aerosur seems to have gone bust but their website is still operational. Suggest you book through kanoo tours if your flying or go direct to the airline company.
In short we were surprised at the excellent bus service provided by trans cococabana. Any questions let me know on firstname.lastname@example.org.