in theory, yes. I can imagine a circumstance in which one might say 'we are seeing the letter'. In practice, it's extremely unlikely. Normally, the meaning of the word 'see' changes whether we say 'see' or 'seeing'.
I see = My eyes see: I see a tree. or metaphorically, I see = I understand. I am seeing = I am dating or visiting: 'I'm seeing a doctor.' Depending on context, that means 'I'm dating a doctor' or 'I am going to a doctor for appointments'. 'I am seeing' can also be used for future plans to watch something: I'm seeing a movie this afternoon.
To say 'we are seeing a letter' probably doesn't mean we are dating a letter or visiting a letter, and letters are something you read, not watch, so it doesn't fall under the usual reasons to use 'seeing' instead of 'see', which makes it sound very odd to use 'we are seeing'. That said, there is one exception to the normal rules: storytelling.
Sometimes we tell stories in present tense to make them more exciting. Present continuous is used to talk about actions that happen before another action:
"We are walking into the room, we are seeing the letter and liking it when suddenly a bear jumps out and eats us!"
So...long story short, yes you can say 'we are seeing the letter' but under normal circumstances that is bad English unless you and the letter are dating. Though to be fair, that would explain why we 'like' it.