Well first of all, you should wait and see whether this is a one-timer. If this is happening more often, discuss with the team, why they are committing to this amount of story points, although it's obvious, that they could handle more.
It's also an option to have a small RnD project, which can be processed in such cases. Perhaps it makes sense due to some strategic goals. It's also a good motivation I think.
To be honest, I've never faced such a scenario, since we have a bug-first policy here, which are usually, unplanned effort and therefore lowering the amount of available feature development time.
As I said in the comments it's optimizing performance by repeating. The best thing of scrum is, that there is an agreement between PM and DEV, which is made on eye level.
This agreement must be adjusted, so that nothing is left at the end and no time is wasted. It's normal, that at the beginning the agreement is not ideal but it will definitely become it sooner or later.
If you're now optimizing from your site by giving them an additional story within the sprint, it's a normal hierarchical situation and it might lower the motivation of the team.
So be patient, and let the team learn its lessons as well as you have to learn yours. Let's say Week 1 the commitment is 30 SP and they are done by Thursday morning. Then in Week 2 the commitment could be 40 SP. Now they couldn't finish. No problem, on Week 3 the commitment should be 35 SP. Performance optimized with 3 weeks.
Of course you might face the issue, that, although you've have found an "ideal" commitment, the result is far from being ideal. But this is also normal, since you're dealing with human beings and not machines.