That's true but if the OP had something along the lines of ''Total cost of job = ££££££s and client will be responsible to incur all additional and unforeseen costs'' That would be watertight
Technically, that would be the common sense approach to it.
The only problem with that being ''you cannot supersede a written contract with a verbal one'' - Any amendment to an initial written contract has to be in writing.
This is not unusual in terms of contractual agreement where you are essentially the ''middle man'' - You are probably correct in that the responsibility of extra costs will fall upon yourself since it would have been you responsibility to liase with suitability of products being used...