Hi @katylou,
I'd say you would need to create item variations which is when you can add an extra cost (I believe it must be the full amount though ie if the item is $20 without your add on, you would make it $22 with the add on if that makes sense).
In terms of capturing the customisation, the only way I think you could do this is in your Online > Settings > Checkout and under 'Customer Inputs' you could enable the 'Allow customer to leave a note to the seller'.
These are by no means perfect solutions, especially if you've got multiple sizes and colours to deal with. If it was me I'd probably add a note to each item that clearly explained that if the customer chooses to customise, they need to specify the customisation at checkout in the 'Note' box or something. Alternatively, you could just capture their order and contact them after (which again isn't perfect).
I'm sure @Seamus can raise this as a feature request to Square though.
Hope that (kind of) helps as an interim work around!
