WooCommerce is what you would use to bridge the gap between your customers and your vender (Printful or Printify). Both of those vendors have pros and cons. What did you find in your own investigation?
If you assume all other things are equal, why market? Traditional marketing is becoming more and more ineffective. "Strong marketing" is growth hacking. No question about it. So if what you heard referred to growth hacking, yes I agree. If they meant - spend more in exposure and cross your fin...
You should try Balsamiq for this. After this you might consider InVision for displaying working and designed solutions