Adding a one off set up fee on top of monthly product subscription cost help ?

Site URL: http://stalbans.thewearecommunity.co.uk

Hello there, I was wondering if anyone could please help..

I'm offering a subscription plan for customers to be featured on the website i'm building. I'd like to add on a one off £5 admin set up fee to be taken off only their first payment. Squarespace can't offer this. 

Does anyone know any way this can be done?

Thanks so much!!

Katie