Add to cart button does not display when using post to page option

When you say “Post to Page” do you mean Attach to Post?
As for the 3 columns add in Nextgen custom CSS area
.ngg-gallery-thumbnail img {margin: 20px 50px;}
or
.ngg-gallery-thumbnail img {margin: 20px 50px !important;}
With regard to the issue, it seems a conflict between the two plugins. You have to test activating default WP theme and deactivating other plugins.
BTW, is there a particular reason for which you can’t use the first method?

Yes, Attach to Post — sorry! I can use the first method but it seems like everything should be working with Attach to Post. Here’s the code that is generated:
<img class=”ngg_displayed_gallery mceItem” alt=”” src=”https://williegreenaldridge.com/wordpress/nextgen-attach_to_post/preview/id–176&#8243; />

<!–[nggallery id=4 template=wp-cart]–>

Thanks for the css. Works beautifully!

@apexbee The first is the NGG code and it’s correct.
The second is a Simple Paypal’s code, built around the NGG legacy shortcode, so you have to ask in that plugin’s support how it works.
Anyway, I read here that the correct example code is [nggallery id=1 template=”wp-cart”].
If you want to use the new ATP function you have to ask to the other plugin’s author if there’s a code working with the current NGG code, this is not a NGG problem.
Otherwise, you will need to use the legacy shortcodes, they are still supported by NGG.

Thanks for your help!