Burn testing in your vessel
Wick size, wax and fragrance load have to be tested in your actual jar. Ask how they test burn performance against ASTM F2417 and how many burn cycles they run.
Safety labels to ASTM F2058
Candles sold in the US are expected to carry fire safety warnings in line with ASTM F2058. Ask whether their labels meet it and whether they review your design.
IFRA-compliant fragrance
Fragrance oils should come with an IFRA certificate showing safe use levels for candles. Ask for it, and for any California Prop 65 review your fragrance needs.
Wax, wick and cure time in writing
Soy, coconut, paraffin and blends behave differently, and most candles need days to cure before they ship. Agree on the wax, wick and cure time per product.
Batch records for wax and fragrance lots
Color, scent throw and frosting change between lots. Each batch should record which wax and fragrance lots went in, so a complaint can be traced.
Minimums per scent and vessel
Minimums are often per scent and per vessel, and custom jars or lids add their own. Get them, and lead times for peak season, in writing.