Step One: the saddlebag
Nudge. vj is six. The lids are stamped in scruples, the smallest thing the apothecary weighs, and the bill is not. Bring the bill down into scruples.
Harder
Three scruples to the drachm. Two drachms is six scruples. Find the line on the bill that says two drachms.
The step, in words
The weight table is three lines: a scruple is twenty grains, a drachm is three scruples, an ounce is eight drachms. The bill lists five things against Sir Anselm Toller's name, each with a weight: wolfsbane root, two drachms; camphor, four drachms; senna leaf, five drachms; rhubarb powder, three drachms; laudanum, one ounce. The five lids are stamped in scruples: nine, twenty-four, six, fifteen, twelve. The lock is a text box labelled The name on the bill; it takes the first word of the line.
The answer
The word is wolfsbane.
The lids are in scruples and the bill is in drachms. Three scruples make a drachm, so two drachms is six scruples: the lid marked vj. Camphor is twelve, senna fifteen, rhubarb nine, and the laudanum is an ounce, twenty-four. Only the wolfsbane comes to six. It was always going to be the wolfsbane; the bill is what makes you sure.