Best Unofficial Squarespace Integrations
Convertkit - ideal for bloggers and creators
Squarespace checkout doesn't allow for much customization. These integrations are useful if you're looking for a customizable checkout option.
Social Proof Apps
Also known as sales notification pop-ups, these apps integrate with your site to show your visitors what actions people have take on your site.
Shipping Fulfillment Services
Memberspace - The number one choice for Squarespace hosted membership sites
SentryLogin - Only $5/mo
MembershipWorks - a web-based membership software system with: event calendar, member directory, online payments, donations, shopping cart, forms, and member only content
Rental Property Booking
Lodgify - fully functioning reservation portal
Equipment and Product Rental
Booqable - Integrates beautifully with Squarespace.
More details on affiliate tracking software that works with Squarespace here.
Tapfiliiate - robust set of features