I don't know why your prices aren't right, but marking things up just so you can mark it down with a fake sale is illegal most places. Customers are smart enough to know if you are always having a perpetual sale that that is what you are doing. It makes them feel you are dishonest and actually avoid your store instead of buying more.
Having sales all the time also trains your regular customers to never buy anything from you unless it is on sale, so you never get full price. You'd do better to just price fairly.
I don't think you should have more than 10% to 20% of your store on sale at any given time, and have sales infrequently unless you are just trying to clear out dead inventory. Then give deep discounts just to get your money out of it so you can invest the money into something more profitable. There is a reason why most brick and morter stores don't have store wide perpetual sales.
I am seeing your sweatshirts at $73.59 regular price and $55.19 at 25% off. The difference you are seeing may be the exchange rate between German marks or Euros or whatever your country uses and the value of a US dollar, which fluctuates all the time.
You can't expect as high a markup on POD products as you would make if you were printing them yourself. You may be setting your prices a bit high to be competitive. And 12 products is barely a drop in the bucket in such a highly competitive niche as POD, so you aren't getting found often enough. And in the US people aren't that keen to buy products made in China. Most people here equate that with poor quality. I would pick one of the more mid range production partners located in either the EU or US. If people don't notice it is made in China and it shows up with Chinese writing on the package, that is just asking for a bad review.
