Get Coupon
GET /self/{shopId}/coupons/{couponId}
curl \
-X GET https://dev.sellpass.io/self/{shopId}/coupons/{couponId} \
-H "Authorization: $API_KEY"
curl \
-X GET https://dev.sellpass.io/self/{shopId}/coupons/{couponId} \
-H "Authorization: $API_KEY"