The peptides were precipitated after further dissolved in 1xPBS to 0.3 mM from the stock of 10 mM in DMSO. May I know what I should do with these peptides?

First, the solubility test report is needed to check whether the peptide could be dissolved in DMSO and PBS.
If no solubility test report, please kindly check the recommended solvent in the COA report.
Then, the sample usually is dissolved in a high concentration in the soluble solvent, and then dilute the samples with the final solvent with a little sample from a high concentration primary solution.