This is a set sentence structure. The structure is 太___了. For example: 太貴了 [it's] too expensive 太大了 [it's] too big 太小了 [it's] too small 太重了 [it's] too heavy It's always correct to use 了 here, although you will see it appear without 了 in some situations. To be on the safe side, I was taught to always use 了.

for emphasis of quality

You can understand the character as a period.

It's an expression to emphasize something/someone too excessive. You could refer to this https://resources.allsetlearning.com/chinese/grammar/Expressing_%22excessively%22_with_%22tai%22

I would assume they already did something nice for you?