Definitely do it in person and take the approach of “I was offered $17.50 initially and I have show I am reliable/hardworking over the last 4 months, and I would like to earn $X now” rather than “it’s not fair I was told this rate but got paid less”. Specific examples to show your value are helpful, use a friendly but firm approach, but recognize usually employers have complete discretion in wages unless you are part of union or have a contract so there’s a chance they will say no.
Another option is to use those 4 months of experience to get a new warehouse job with better pay. People tend to earn more job hopping than waiting for raises.