I realize I’m not the member your comment is directed to, but adding my two cents worth anyway and in case it’s a general question since you are newer to Postcrossing.
For sending postcards within the USA, using a forever stamp, instead of the usps postcard stamp, is a fun way to get more variety. As the cost of postage continues to go up here in the US, using forever stamps on postcards can be too expensive for those on a budget.
As for sending international postcards, there’s the one round international usps stamp, that is a good one to use on cards where there’s not much space. Otherwise any combination of stamps can be used as long as it adds up to the international stamp value, including forever stamps, counted at the current first class card rate(which is going up again in July).
I don’t want to get off topic here, so adding some links to other forum topics specifically about stamps and postage.
and in case it’s a general question since you are newer to Postcrossing.