The break even point is probably closer to 50 cards than 1000. The service you recommended costs an extra $300USD per 100 cards more than doing it yourself.
For 50 cards @ a service charges $3 extra per card is $150 - extra assuming no additional taxes or charges from them. Shipping is less than that, PSA charges $75 return shipping internationally ($55 more than domestic).
The customs charge should be 0 because you’re getting your own items returned back to you but if for some that’s not true for Germany, you’re then obligated to pay customs whether you ship yourself or through a middleman service. If you ship yourself, you have full control over declared value in both directions.
Not to mention that if things ever go bad with your middleman company you have basically no control or recourse: Graded Gem - UK Graders - No communication / Outstanding PSA Orders with Ludkins Collectables USA
Overall, unless you’re sending in a trivially small number of card, like 1-10, it’s probably worth doing it yourself regardless of where you live.