スーパーに行ったら、牛乳をかってきてくれる?
The Bunpro Translation of the above is:
If you go to the supermarket, could you buy some milk (and come back)? [for me]
Since we're asking for a favor (it sounds like we are), then why isn't kureru in the Te form?
牛乳をかってきてくれる?
The pattern is て+くれる
, which is also applicable to verbs such as あげる and やる.
By using くれる, it's implied that the speaker is requesting a favor, so there's no need to use its て form or to even pair it with ください.
Related: Is くれてください tautological?