You can always use a simple syrup on your cakes (with or without additional flavoring), just make sure not to OVER use the syrup (it will sometimes leak out of the cake). I use syrup on my cakes to help with keeping them moist but don't overdo it. Unless your cakes are really dry, it won't need it.
Also, per Sharon's video, make sure to let the cake sit and for the outside to dry. I will actually try to blot some moisture off of the outside of the cake to help with the bubble affect, which I tend to get bubbles in my frosting for some reason so I try to make sure it gets to room temp at some point before delivery so I can fix it at home if there is a bubble issue.
Also, I just use strawberry jelly, works well especially if you can find a brand that has some chunks of strawberries in it. It may be more affordable than buying the filling in the sleeves -- unless you prefer that, of course.