The premise of your question is wrong. This is because there is no future tense in English. We can refer to future time in many ways. One way is to use the simple present, as in:
1 Unless I see the money on my desk by tomorrow 9am, you're a dead man.
The modal will can also be used to refer to future time. This seems to be the construction you're asking about, so I'll bold an example:
2 Unless they will agree to paint the house red, do not hire them.
(It has been stated that "(this usage of will) is not exactly about the future; it's about volition, i.e. "will" (or willingness)." To which, I reply that as uttered by someone, I am not sure we can neatly cleave willingness and referring to future time. I will go with you is expressing one's present willingness to perform the stated action in the future). Even so, I offer another example:
3 Unless they'll go to see Star Wars tomorrow, I'm not going with them.
But so can about ten other constructions, including these common ones:
4 Unless I am seeing the money on my desk in five minutes, you're a dead man.
5 Unless you are going to tell me the real reason, don't bother to open your mouth.
6 I don't want to bother you for a ride to Boston, unless you'll be driving there anyway.
7 Unless you're about to leave, don't bother washing the car windows.
8 We're going to have way too much work to do unless you are to come to your senses and hire some temps.
(Granted the last construction {to be plus to} is rather stilted and part of 'old school' business language.)
Edited to add:
Other examples of present progressive to refer to future time:
9 Unless I'm sitting here and hearing the sound of your feet walking up the stairs to my office in five minutes, don't bother to come.
10 You can't have any unless you're doing your homework in five minutes.
Also note the response to the question Can I have some candy?:
11 No you may not. Not unless you're doing your homework in five minutes.*
Etc.
Best Answer
Please let me know when you have shipped them is unambiguous but requires the shipper to understand " when you have shipped them".
"Please notify me when they have been shipped" is unambiguous but requires the shipper to understand "when they have been shipped".
Please send a postal tracking number if available, or a shipping notice with ETA uses nouns common to international online retail, so it is more likely to be understood than any sentence whose meaning depends on the verb tense, or on the shipper's understanding a phrase such as "at the time".