The Japa-samarpana mantras according to Shyamarahasya(page 3) for a male deity is
guhyAtiguhya-goptA tvam grihaNasmatkrtam japam/ siddhir bhavatu me deva tvat prasAdat tvayi sthite//
For a female deity the mantra is
guhyAtiguhyagoptri tvam grihANAsmatkritam japam/siddhir bhavatu me devi tvatprasAdAt tvayi sthite//
In some books, instead of the last two words, "Mahesvara' and 'Mahesvari' are also seen.
According to the Purascharana-chandrika, if the method of COUNTING is not followed properly, the rakshasas steal the fruits of the japa and so intelligent people must count properly:
gaNanAvidhim ullanghya yo japet tu prmAditah/ grihnananti rAkshasAh yasmat niyatam ganayet budhah//
The counting is just for that time.Not for remembering life long.Sadhak Kamalakanta, a famous shAkata saint says
If you protect by Your Own goodness and give a merciful grance, only then sadhana is fruitful. Getting your vision by doing japa is absurd like the marriage of the ghosts.
But that dependence and surrender is the ultimate.In the beginning, an aspirant should follow the injunctions of the scriptures to the best of his/her abilities. So the achamana, asana, place, counting, samarpana etc-- each is important. Regarding samarpana of japa, PurascharyarNava says
evam japam pura kritva gandha-akshata-kusha-udakaih/japam samarpayed devyah vamahaste vichakshaNah// meaning that an intelligent individual should, after completing the japa, offer the japa to the left hand of Devi with sandal paste, atapa rice, kusha-grass and (sacred) water.
For Male Deity, to right hand is japa offered with sandal paste, flower,mand water as Arghya:
kritva japam pura chaivam tejorupam samarpayet/ Devasya dakshiNe haste kusha-pushpa-arghya-vAribhih (Sanatkumara-Tantravachana)
After japa, pradakshina and then pranAmaa are also obligatory:
tatas cha devatAm bhaktyA parikramya named budhah: YAmala-vachana.
As Sri Krishna says in the Gita
16.23 Ignoring the precept of the scriptures, he who acts under the impulsion of passion,-he does not attain perfection, nor happiness, nor the supreme Goal.
16.24 Therefore, the scripture is your authority as regards the determination of what is to be done and what is not to be done. After understanding (your) duty as presented by scriptural injunction, you ought to perform (your duty) here.
I never heard that the fruits 'go to Indra'! Please note than Indra is one major form of God in the Vedas.In fact, One should remember that His/Her Ishtadevata has assumed the differnt forms Himself/Herself.But to get the fruits of japa, one has to do the samarpana properly.
Only for manasa-japa, NO counting is required and it is the best of the japas.It does not need asana or restrictions on the place etc either.
(Reference: Sastramulak Bharatiya shaktisadhana,mUpendrakumar Das,RMIC, vol 2, chapter :japa)