Hmm I think you should be free to choose which den you want to add a spot to. If you first want to do the cheapest that is of course everyone's perogative, but personally I prefer expanding one den to the max before moving onto the next one. Perhaps a pop up saying; Your den is full, please select a den to purchase a new slot from (enter dropdown list)

That might work better! I try to expand in slots of 5 because I dislike uneven dens.

Support! I just had my heart broken because the first WW in a month decided to try and come home with me... only for my den to be full because i had forgotten about a litter of four pups that had taken up the last few slots i had.

SUPPORT

this is one of the worst feel bads i have ever experienced on a pet site as a new player and made me so sad : (

i like the buying a den slot the best, but i had some alternative solutions in case that doesn't work:

a. dump them into a waiting zone that they will fill the next available slot

b. make it so the next wild wolf you encounter also agrees to join you guaranteed

c. give you at LEAST a knuckle bone come on!!

Support! As rare as WW are, it feels unfair to nit realize you don't have space until it's too late