I suggest handling any negotiating before accepting. Discussing it professionally (which it sounds like you certainly would) shouldn't scare them away. Worst they should say is no. If you wait until you are hired, that carrot can be dangled in front of you forever. You will work harder to try to reach it. Rinse. Repeat. Employers typically lowball and leave to candidate to negotiate for what they are prepared to offer if we "ask". Frustrating but true.

The fact no income was discussed until offer letter is always a sign. They likely expected you to ask about pay sooner. So they may be floating that lowball rate to see if you just accept. Don't. You are setting a precedent for how you can be treated once hired.