To do this you are going to need to make use of two tags. Now this is a little fiddly to describe here but I will try and be as clear as possible
You need to start the campaign with the tag your POS has applied and feed this in to a sequence.
Inside this sequence you need to remove the tag your POS has applied (allowing it to recirculate).
You then need to feed this sequence in to two further sequences to create a decision diamond.
In one of these sequences you want to apply one further tag called something like “Recent Purchase”. you then want to send an email requesting feedback asking for ratings or reviews. You then (still inside this sequence) want to add a timer for whatever length of time you want as a minimum between sending out these emails. Then feed that timer in to a remove tag to remove your “Recent Purchase” tag.
What we have done here is set up a system that means that the “Recent Purchase” tag will be present on a contact for, in your case, 6 weeks.
Now what you want to do is come back to the main campaign screen and select the decision diamond you created earlier. Set this to funnel customers in to the sequence you have just created if they do not have the “Recent Purchase” tag. Set the people who DO have this tag to filter off in to your empty sequence.
This means that your email will only be sent to people who have not made a repeat purchase within the last 6 weeks, and that people who have are not sent anything.
Hope this is clear enough to clear it up!