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Please allow me to help you. Based on the title of your post and what I've seen on the form URL you provided, you would like to allow users to see the available sizes of the t-shirts once they've selected the "purchase optional shirt". If I've understood you correctly, I was hoping I did, you may follow these simple instructions I've created.
1. Open your form URL for you to edit it and select "Conditions" from one of the icons in the menu. Make sure you are in the "Setup & Embed tab" so the "Conditions" icon will show.
2. Once you have clicked "Conditions" this window will appear. Then, select "Show/Hide a form field" from the list of options and click "Next" on the bottom right corner.
3. Once you've clicked "Next" this window will appear where you can create your conditions.
Then create your first condition and click save.
You need to repeat steps 1-3, so you can create your second condition as shown below. Make sure you click save after.
Once you're done adding the first and the second condition, you should have this window bellow when you click the "Conditions" icon again from the menu and then click "Saved Conditions" from the bottom left corner of the window that appeared.
And when these two conditions are already saved in your form. You can now check if it's working.
Here's what will happen and I hope this is what you want as an output.
In addition to what Mike had suggested, there is also a way to add sub-option to the main item/product on the payment form.
Check this user guide:
The sub-option item is viewable at all times, but if the end-user will not select the specific item, it will not be included in the purchase just the same.
Let us know if you need further assistance.