98 % versus 99 %

The last two percent is the expensive part, because the impurities that remain are structurally similar to the product. Going from 95 % to 98 % may double the purification burden; 98 % to 99 % can double it again.

Before specifying 99 %, check three things: whether the assay your customer uses can distinguish it, whether the impurity at that level matters for the application, and whether your specification names the method. “≥ 98.0 % by RP-HPLC, C18, 214 nm, threshold 0.1 %” is a specification. “≥ 98 %” is an invitation to argue.

Also separate purity by area from peptide content by mass. A 98 % pure acetate salt can be 80 % peptide by weight once water and counter-ion are counted. If you buy by mass, you are buying the salt.