I too was unaware there was a commercial liquid suspension available, given that fenbenzadole is non-soluble. I have always worked with the powder and created suspensions to administer doses.
I am a cheapskate so I always purchased the "horse" sized packet online. I never used the powder for dusting of food insects as the insolubility results in a risk of blocking breathing passages.(Keep in mind I was working with true chameleons!) For small specimens I always "inflated" a properly sized cricket with a liquid suspension of roughly one milligram to 30 milliliters of water.
With my amphibians, I still use the cricket method whenever possible. On large amphibians, I administer the dose orally dependent on body weight using a graduated plastic pipette.
As to the safety of the drug to amphibians, well, last I heard the information was non-existent.
As Panacur is an effective treatment against Hydroids in aquaria, this topic came up well over a year ago on Caudata.org. I have since only used Panacur in cases where I was absolutely sure of a worm infection as a result of a positive fecal analysis. (I do get bored of poking through tiny turds under a stereo microscope!)