Look at that closely with the brower’s inspector. There is an empty h4 above the h3 that has a big top margin on it. That h4 shouldn’t be there to begin with. Or at least it shouldn’t be before the h3.
I believe the question may be out of scope of this forum. Without having access to the back-end of WP website with the plugin (to be clear I am not asking to share credentials), we cannot see what possibilities the plugin offers. It may also be caused by the setup of your WP website which requires an h4 in that block.
I think you may have better luck on the WP forum as there are higher chances someone else is using that plugin (not all of us use WP).
The plugin widget only has a title and text input, which is an h4 and a p element. Your title element is empty and you have an h3. The h3 seems to be part of the theme, not the plugin.
If you can remove the h3 and use the plugin title you might still need to change the top margin.
.wp-subscribe-single #wp-subscribe .title {
/* set to 0 or however much margin you want */
margin-top: 10px
}
If you can’t get rid of the h3 and use the widget title, one option is to use a negative margin on the h3
Both selectors will target all plugin instances. If you want to only target the one on the specific post you might try adding the post id in front of the selector. Although, I’m not sure how stable the id is (updating the page might update the id).