Adding Subscription Option as a Product Variant

I'm trying to offer my clients the option to pay for a service upfront or in monthly instalments. Is it possible to only apply a subscription to a variant rather than the product as a whole?

It depends what you mean by pay in installments.

If you are using AfterPay to allow customers to pay in installments, this option will appear at checkout, giving customers the option to pay in full or use AfterPay (subject to their approval).

If you are referring to Subscription Products, these are managed separately on Squarespace and don't even use the same checkout. If a Subscription Product is added to the cart, the customer will be forced to checkout with this separately. For this reason, Subscription Products must be separate. It isn't possible to add a subscription variant to a Physical or Service Product.

However, if you have a Subscription Product and a separate Physical/Service Product, you could use some custom code to display the subscription option as a button beside the standard add to cart button, so that customers see 'One Time Purchase' and 'Subscribe and Save' options together.

Site URL: http://drinkdroplet.com

Squarespace— 

We just need a native function that allows customers to choose either a one-time purchase or a subscription on the same product page. Currently, we have to list subscriptions and one-time purchases separately, and any marketer can tell you that this leads to less conversions. 

This needs to be incorporated into your next updates for the commerce site. Those of us who subscribe to and develop for sites on your Advanced Commerce plans are paying extra to be able to use subscriptions and this function alone shows that Squarespace Commerce is not truly optimized for sales. 

Here is a list of all the times people have asked for this particular function: 

https://www.google.com/search?q=squarespace+subscription+or+one+time+purchase+site:forum.squarespace.com&sa=X&ved=2ahUKEwji3efTsbbwAhXYFzQIHf_6ARIQrQIoBHoECAUQBQ&biw=1680&bih=916

When is this feature going to be added? 

you need to log this as a feature request by creating a ticket. Squarespace doesn't monitor this forum, per their rules, for feature requests.