We have several add-ons that do not add time to a service. The add-on blocks being added at the end of each session is very odd visually for our front desk team and for the massage therapists - it's easy to miss for the service provider when back to back (looks like it's for the following client) and at a glance looks intuitively like it's blocking the schedule time (even though it's not). For us it works much better to have the block run parallel/overlapping with the service at the top.