Okay. That is good.
Have you verified the incoming data by echoing it to the Serial Monitor?
Is your receiving system parsing the commas as needed components of the position command (you saw "s,90" sent the "s" servo to zero before 90 due to the extra comma. The sender and receiver should remove the extra comma).
Also, character arrays (c-strings) need a "null terminator" (\0) inside the data being sent to indicate the end of the string. Without the null, reading beyond that memory location will cause unpredictable results.
You should add a limit checking routine as the data leaves the sender and arrives at the receiver.