I agree with the earlier comment; I think this phrase is too informal for an email to customers.
That said, I think this expression would be fine in an email to coworkers and colleagues, where a degree of informality is more acceptable. However, I'd probably use a dash instead of a comma:
If you have any feature suggestion then let me know – I'm all ears.
or I would leave out the "let me know" part, because that seems to be a bit redundant in this context:
If you have any feature suggestion, I'm all ears.
Back to your email to the customers – I think you are trying to convey an eagerness for their input. This could be accomplished by adding one word – the word please – to your initial statement:
If you have any feature suggestion then please let me know.