Another, and cleaner, approach is to use IAP and sell it all within the app. If necessary, increase you prices by 30% so you net the same.
Regarding all your IAP issues......(except, perhaps, the discounting)
As I suggested above - You can sell a package of 'tokens' as a consumable IAP. The user can then exchange those tokens for various products from a list of products within the app. Each product could require a different number of 'tokens'. You can sell a package of 1,000,000 tokens for $6.99 as a consumable IAP. You can sell a package of 10 tokens for $12.99. Each of your models can then list their 'price' in tokens. At checkout you add the number of tokens and deduct it from the user's token count.
The guidelines state:
Any credits or in-game currencies purchased via in-app purchase may not expire, and you should make sure you have a restore mechanism for any restorable in-app purchases.
If you do this:
>With IAP, i can't let user to buy few item at once.
Yes you can.
>limit of consumable item in in-app purchase method is 10.000
Not true. And in the above example, you can sell "1,000,000 tokens" in one purchase
> Items price can be changed during some events
You could have the app reference, in the background, a price list of 'number of tokens for each model' that you would maintain on a website somewhere. The app would read that list and change the price on a moments notice. You could even have bidding adjusted prices.
>IAP can send request only for one consumable item at the same time.
Not true with the token approach
> I'm providing discount codes,
This borders on a violation of the guidelines in that it enables your ability to 'sell a discount' outside the app and transfer the discount into the app ("Apps may not use their own mechanisms to unlock content or functionality"). But if App Review choses not to worry about your doing that, and discounting is not considered by App Review as a violation, you could use a device-specific code and your background website price list to discount a product for a specific user on a specific device.
